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Diffstat (limited to '')
| -rw-r--r-- | kernel/locking/rwsem.c | 1694 |
1 files changed, 1694 insertions, 0 deletions
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c new file mode 100644 index 000000000..cc5cc889b --- /dev/null +++ b/kernel/locking/rwsem.c @@ -0,0 +1,1694 @@ +// SPDX-License-Identifier: GPL-2.0 +/* kernel/rwsem.c: R/W semaphores, public implementation + * + * Written by David Howells (dhowells@redhat.com). + * Derived from asm-i386/semaphore.h + * + * Writer lock-stealing by Alex Shi <alex.shi@intel.com> + * and Michel Lespinasse <walken@google.com> + * + * Optimistic spinning by Tim Chen <tim.c.chen@intel.com> + * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes. + * + * Rwsem count bit fields re-definition and rwsem rearchitecture by + * Waiman Long <longman@redhat.com> and + * Peter Zijlstra <peterz@infradead.org>. + */ + +#include <linux/types.h> +#include <linux/kernel.h> +#include <linux/sched.h> +#include <linux/sched/rt.h> +#include <linux/sched/task.h> +#include <linux/sched/debug.h> +#include <linux/sched/wake_q.h> +#include <linux/sched/signal.h> +#include <linux/sched/clock.h> +#include <linux/export.h> +#include <linux/rwsem.h> +#include <linux/atomic.h> + +#include "lock_events.h" + +/* + * The least significant 3 bits of the owner value has the following + * meanings when set. + * - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers + * - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock. + * - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock. + * + * When the rwsem is either owned by an anonymous writer, or it is + * reader-owned, but a spinning writer has timed out, both nonspinnable + * bits will be set to disable optimistic spinning by readers and writers. + * In the later case, the last unlocking reader should then check the + * writer nonspinnable bit and clear it only to give writers preference + * to acquire the lock via optimistic spinning, but not readers. Similar + * action is also done in the reader slowpath. + + * When a writer acquires a rwsem, it puts its task_struct pointer + * into the owner field. It is cleared after an unlock. + * + * When a reader acquires a rwsem, it will also puts its task_struct + * pointer into the owner field with the RWSEM_READER_OWNED bit set. + * On unlock, the owner field will largely be left untouched. So + * for a free or reader-owned rwsem, the owner value may contain + * information about the last reader that acquires the rwsem. + * + * That information may be helpful in debugging cases where the system + * seems to hang on a reader owned rwsem especially if only one reader + * is involved. Ideally we would like to track all the readers that own + * a rwsem, but the overhead is simply too big. + * + * Reader optimistic spinning is helpful when the reader critical section + * is short and there aren't that many readers around. It makes readers + * relatively more preferred than writers. When a writer times out spinning + * on a reader-owned lock and set the nospinnable bits, there are two main + * reasons for that. + * + * 1) The reader critical section is long, perhaps the task sleeps after + * acquiring the read lock. + * 2) There are just too many readers contending the lock causing it to + * take a while to service all of them. + * + * In the former case, long reader critical section will impede the progress + * of writers which is usually more important for system performance. In + * the later case, reader optimistic spinning tends to make the reader + * groups that contain readers that acquire the lock together smaller + * leading to more of them. That may hurt performance in some cases. In + * other words, the setting of nonspinnable bits indicates that reader + * optimistic spinning may not be helpful for those workloads that cause + * it. + * + * Therefore, any writers that had observed the setting of the writer + * nonspinnable bit for a given rwsem after they fail to acquire the lock + * via optimistic spinning will set the reader nonspinnable bit once they + * acquire the write lock. Similarly, readers that observe the setting + * of reader nonspinnable bit at slowpath entry will set the reader + * nonspinnable bits when they acquire the read lock via the wakeup path. + * + * Once the reader nonspinnable bit is on, it will only be reset when + * a writer is able to acquire the rwsem in the fast path or somehow a + * reader or writer in the slowpath doesn't observe the nonspinable bit. + * + * This is to discourage reader optmistic spinning on that particular + * rwsem and make writers more preferred. This adaptive disabling of reader + * optimistic spinning will alleviate the negative side effect of this + * feature. + */ +#define RWSEM_READER_OWNED (1UL << 0) +#define RWSEM_RD_NONSPINNABLE (1UL << 1) +#define RWSEM_WR_NONSPINNABLE (1UL << 2) +#define RWSEM_NONSPINNABLE (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE) +#define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE) + +#ifdef CONFIG_DEBUG_RWSEMS +# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \ + if (!debug_locks_silent && \ + WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\ + #c, atomic_long_read(&(sem)->count), \ + (unsigned long) sem->magic, \ + atomic_long_read(&(sem)->owner), (long)current, \ + list_empty(&(sem)->wait_list) ? "" : "not ")) \ + debug_locks_off(); \ + } while (0) +#else +# define DEBUG_RWSEMS_WARN_ON(c, sem) +#endif + +/* + * On 64-bit architectures, the bit definitions of the count are: + * + * Bit 0 - writer locked bit + * Bit 1 - waiters present bit + * Bit 2 - lock handoff bit + * Bits 3-7 - reserved + * Bits 8-62 - 55-bit reader count + * Bit 63 - read fail bit + * + * On 32-bit architectures, the bit definitions of the count are: + * + * Bit 0 - writer locked bit + * Bit 1 - waiters present bit + * Bit 2 - lock handoff bit + * Bits 3-7 - reserved + * Bits 8-30 - 23-bit reader count + * Bit 31 - read fail bit + * + * It is not likely that the most significant bit (read fail bit) will ever + * be set. This guard bit is still checked anyway in the down_read() fastpath + * just in case we need to use up more of the reader bits for other purpose + * in the future. + * + * atomic_long_fetch_add() is used to obtain reader lock, whereas + * atomic_long_cmpxchg() will be used to obtain writer lock. + * + * There are three places where the lock handoff bit may be set or cleared. + * 1) rwsem_mark_wake() for readers. + * 2) rwsem_try_write_lock() for writers. + * 3) Error path of rwsem_down_write_slowpath(). + * + * For all the above cases, wait_lock will be held. A writer must also + * be the first one in the wait_list to be eligible for setting the handoff + * bit. So concurrent setting/clearing of handoff bit is not possible. + */ +#define RWSEM_WRITER_LOCKED (1UL << 0) +#define RWSEM_FLAG_WAITERS (1UL << 1) +#define RWSEM_FLAG_HANDOFF (1UL << 2) +#define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1)) + +#define RWSEM_READER_SHIFT 8 +#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT) +#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1)) +#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED +#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK) +#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\ + RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL) + +/* + * All writes to owner are protected by WRITE_ONCE() to make sure that + * store tearing can't happen as optimistic spinners may read and use + * the owner value concurrently without lock. Read from owner, however, + * may not need READ_ONCE() as long as the pointer value is only used + * for comparison and isn't being dereferenced. + */ +static inline void rwsem_set_owner(struct rw_semaphore *sem) +{ + atomic_long_set(&sem->owner, (long)current); +} + +static inline void rwsem_clear_owner(struct rw_semaphore *sem) +{ + atomic_long_set(&sem->owner, 0); +} + +/* + * Test the flags in the owner field. + */ +static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags) +{ + return atomic_long_read(&sem->owner) & flags; +} + +/* + * The task_struct pointer of the last owning reader will be left in + * the owner field. + * + * Note that the owner value just indicates the task has owned the rwsem + * previously, it may not be the real owner or one of the real owners + * anymore when that field is examined, so take it with a grain of salt. + * + * The reader non-spinnable bit is preserved. + */ +static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem, + struct task_struct *owner) +{ + unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED | + (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE); + + atomic_long_set(&sem->owner, val); +} + +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) +{ + __rwsem_set_reader_owned(sem, current); +} + +/* + * Return true if the rwsem is owned by a reader. + */ +static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem) +{ +#ifdef CONFIG_DEBUG_RWSEMS + /* + * Check the count to see if it is write-locked. + */ + long count = atomic_long_read(&sem->count); + + if (count & RWSEM_WRITER_MASK) + return false; +#endif + return rwsem_test_oflags(sem, RWSEM_READER_OWNED); +} + +#ifdef CONFIG_DEBUG_RWSEMS +/* + * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there + * is a task pointer in owner of a reader-owned rwsem, it will be the + * real owner or one of the real owners. The only exception is when the + * unlock is done by up_read_non_owner(). + */ +static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem) +{ + unsigned long val = atomic_long_read(&sem->owner); + + while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) { + if (atomic_long_try_cmpxchg(&sem->owner, &val, + val & RWSEM_OWNER_FLAGS_MASK)) + return; + } +} +#else +static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem) +{ +} +#endif + +/* + * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag + * remains set. Otherwise, the operation will be aborted. + */ +static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem) +{ + unsigned long owner = atomic_long_read(&sem->owner); + + do { + if (!(owner & RWSEM_READER_OWNED)) + break; + if (owner & RWSEM_NONSPINNABLE) + break; + } while (!atomic_long_try_cmpxchg(&sem->owner, &owner, + owner | RWSEM_NONSPINNABLE)); +} + +static inline bool rwsem_read_trylock(struct rw_semaphore *sem) +{ + long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count); + if (WARN_ON_ONCE(cnt < 0)) + rwsem_set_nonspinnable(sem); + return !(cnt & RWSEM_READ_FAILED_MASK); +} + +/* + * Return just the real task structure pointer of the owner + */ +static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem) +{ + return (struct task_struct *) + (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK); +} + +/* + * Return the real task structure pointer of the owner and the embedded + * flags in the owner. pflags must be non-NULL. + */ +static inline struct task_struct * +rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags) +{ + unsigned long owner = atomic_long_read(&sem->owner); + + *pflags = owner & RWSEM_OWNER_FLAGS_MASK; + return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK); +} + +/* + * Guide to the rw_semaphore's count field. + * + * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned + * by a writer. + * + * The lock is owned by readers when + * (1) the RWSEM_WRITER_LOCKED isn't set in count, + * (2) some of the reader bits are set in count, and + * (3) the owner field has RWSEM_READ_OWNED bit set. + * + * Having some reader bits set is not enough to guarantee a readers owned + * lock as the readers may be in the process of backing out from the count + * and a writer has just released the lock. So another writer may steal + * the lock immediately after that. + */ + +/* + * Initialize an rwsem: + */ +void __init_rwsem(struct rw_semaphore *sem, const char *name, + struct lock_class_key *key) +{ +#ifdef CONFIG_DEBUG_LOCK_ALLOC + /* + * Make sure we are not reinitializing a held semaphore: + */ + debug_check_no_locks_freed((void *)sem, sizeof(*sem)); + lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP); +#endif +#ifdef CONFIG_DEBUG_RWSEMS + sem->magic = sem; +#endif + atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE); + raw_spin_lock_init(&sem->wait_lock); + INIT_LIST_HEAD(&sem->wait_list); + atomic_long_set(&sem->owner, 0L); +#ifdef CONFIG_RWSEM_SPIN_ON_OWNER + osq_lock_init(&sem->osq); +#endif +} +EXPORT_SYMBOL(__init_rwsem); + +enum rwsem_waiter_type { + RWSEM_WAITING_FOR_WRITE, + RWSEM_WAITING_FOR_READ +}; + +struct rwsem_waiter { + struct list_head list; + struct task_struct *task; + enum rwsem_waiter_type type; + unsigned long timeout; + unsigned long last_rowner; +}; +#define rwsem_first_waiter(sem) \ + list_first_entry(&sem->wait_list, struct rwsem_waiter, list) + +enum rwsem_wake_type { + RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */ + RWSEM_WAKE_READERS, /* Wake readers only */ + RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ +}; + +enum writer_wait_state { + WRITER_NOT_FIRST, /* Writer is not first in wait list */ + WRITER_FIRST, /* Writer is first in wait list */ + WRITER_HANDOFF /* Writer is first & handoff needed */ +}; + +/* + * The typical HZ value is either 250 or 1000. So set the minimum waiting + * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait + * queue before initiating the handoff protocol. + */ +#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250) + +/* + * Magic number to batch-wakeup waiting readers, even when writers are + * also present in the queue. This both limits the amount of work the + * waking thread must do and also prevents any potential counter overflow, + * however unlikely. + */ +#define MAX_READERS_WAKEUP 0x100 + +/* + * handle the lock release when processes blocked on it that can now run + * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must + * have been set. + * - there must be someone on the queue + * - the wait_lock must be held by the caller + * - tasks are marked for wakeup, the caller must later invoke wake_up_q() + * to actually wakeup the blocked task(s) and drop the reference count, + * preferably when the wait_lock is released + * - woken process blocks are discarded from the list after having task zeroed + * - writers are only marked woken if downgrading is false + */ +static void rwsem_mark_wake(struct rw_semaphore *sem, + enum rwsem_wake_type wake_type, + struct wake_q_head *wake_q) +{ + struct rwsem_waiter *waiter, *tmp; + long oldcount, woken = 0, adjustment = 0; + struct list_head wlist; + + lockdep_assert_held(&sem->wait_lock); + + /* + * Take a peek at the queue head waiter such that we can determine + * the wakeup(s) to perform. + */ + waiter = rwsem_first_waiter(sem); + + if (waiter->type == RWSEM_WAITING_FOR_WRITE) { + if (wake_type == RWSEM_WAKE_ANY) { + /* + * Mark writer at the front of the queue for wakeup. + * Until the task is actually later awoken later by + * the caller, other writers are able to steal it. + * Readers, on the other hand, will block as they + * will notice the queued writer. + */ + wake_q_add(wake_q, waiter->task); + lockevent_inc(rwsem_wake_writer); + } + + return; + } + + /* + * No reader wakeup if there are too many of them already. + */ + if (unlikely(atomic_long_read(&sem->count) < 0)) + return; + + /* + * Writers might steal the lock before we grant it to the next reader. + * We prefer to do the first reader grant before counting readers + * so we can bail out early if a writer stole the lock. + */ + if (wake_type != RWSEM_WAKE_READ_OWNED) { + struct task_struct *owner; + + adjustment = RWSEM_READER_BIAS; + oldcount = atomic_long_fetch_add(adjustment, &sem->count); + if (unlikely(oldcount & RWSEM_WRITER_MASK)) { + /* + * When we've been waiting "too" long (for writers + * to give up the lock), request a HANDOFF to + * force the issue. + */ + if (!(oldcount & RWSEM_FLAG_HANDOFF) && + time_after(jiffies, waiter->timeout)) { + adjustment -= RWSEM_FLAG_HANDOFF; + lockevent_inc(rwsem_rlock_handoff); + } + + atomic_long_add(-adjustment, &sem->count); + return; + } + /* + * Set it to reader-owned to give spinners an early + * indication that readers now have the lock. + * The reader nonspinnable bit seen at slowpath entry of + * the reader is copied over. + */ + owner = waiter->task; + if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) { + owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE); + lockevent_inc(rwsem_opt_norspin); + } + __rwsem_set_reader_owned(sem, owner); + } + + /* + * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the + * queue. We know that the woken will be at least 1 as we accounted + * for above. Note we increment the 'active part' of the count by the + * number of readers before waking any processes up. + * + * This is an adaptation of the phase-fair R/W locks where at the + * reader phase (first waiter is a reader), all readers are eligible + * to acquire the lock at the same time irrespective of their order + * in the queue. The writers acquire the lock according to their + * order in the queue. + * + * We have to do wakeup in 2 passes to prevent the possibility that + * the reader count may be decremented before it is incremented. It + * is because the to-be-woken waiter may not have slept yet. So it + * may see waiter->task got cleared, finish its critical section and + * do an unlock before the reader count increment. + * + * 1) Collect the read-waiters in a separate list, count them and + * fully increment the reader count in rwsem. + * 2) For each waiters in the new list, clear waiter->task and + * put them into wake_q to be woken up later. + */ + INIT_LIST_HEAD(&wlist); + list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) { + if (waiter->type == RWSEM_WAITING_FOR_WRITE) + continue; + + woken++; + list_move_tail(&waiter->list, &wlist); + + /* + * Limit # of readers that can be woken up per wakeup call. + */ + if (woken >= MAX_READERS_WAKEUP) + break; + } + + adjustment = woken * RWSEM_READER_BIAS - adjustment; + lockevent_cond_inc(rwsem_wake_reader, woken); + if (list_empty(&sem->wait_list)) { + /* hit end of list above */ + adjustment -= RWSEM_FLAG_WAITERS; + } + + /* + * When we've woken a reader, we no longer need to force writers + * to give up the lock and we can clear HANDOFF. + */ + if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF)) + adjustment -= RWSEM_FLAG_HANDOFF; + + if (adjustment) + atomic_long_add(adjustment, &sem->count); + + /* 2nd pass */ + list_for_each_entry_safe(waiter, tmp, &wlist, list) { + struct task_struct *tsk; + + tsk = waiter->task; + get_task_struct(tsk); + + /* + * Ensure calling get_task_struct() before setting the reader + * waiter to nil such that rwsem_down_read_slowpath() cannot + * race with do_exit() by always holding a reference count + * to the task to wakeup. + */ + smp_store_release(&waiter->task, NULL); + /* + * Ensure issuing the wakeup (either by us or someone else) + * after setting the reader waiter to nil. + */ + wake_q_add_safe(wake_q, tsk); + } +} + +/* + * This function must be called with the sem->wait_lock held to prevent + * race conditions between checking the rwsem wait list and setting the + * sem->count accordingly. + * + * If wstate is WRITER_HANDOFF, it will make sure that either the handoff + * bit is set or the lock is acquired with handoff bit cleared. + */ +static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, + enum writer_wait_state wstate) +{ + long count, new; + + lockdep_assert_held(&sem->wait_lock); + + count = atomic_long_read(&sem->count); + do { + bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF); + + if (has_handoff && wstate == WRITER_NOT_FIRST) + return false; + + new = count; + + if (count & RWSEM_LOCK_MASK) { + if (has_handoff || (wstate != WRITER_HANDOFF)) + return false; + + new |= RWSEM_FLAG_HANDOFF; + } else { + new |= RWSEM_WRITER_LOCKED; + new &= ~RWSEM_FLAG_HANDOFF; + + if (list_is_singular(&sem->wait_list)) + new &= ~RWSEM_FLAG_WAITERS; + } + } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)); + + /* + * We have either acquired the lock with handoff bit cleared or + * set the handoff bit. + */ + if (new & RWSEM_FLAG_HANDOFF) + return false; + + rwsem_set_owner(sem); + return true; +} + +#ifdef CONFIG_RWSEM_SPIN_ON_OWNER +/* + * Try to acquire read lock before the reader is put on wait queue. + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff + * is ongoing. + */ +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem) +{ + long count = atomic_long_read(&sem->count); + + if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF)) + return false; + + count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count); + if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) { + rwsem_set_reader_owned(sem); + lockevent_inc(rwsem_opt_rlock); + return true; + } + + /* Back out the change */ + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); + return false; +} + +/* + * Try to acquire write lock before the writer has been put on wait queue. + */ +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) +{ + long count = atomic_long_read(&sem->count); + + while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) { + if (atomic_long_try_cmpxchg_acquire(&sem->count, &count, + count | RWSEM_WRITER_LOCKED)) { + rwsem_set_owner(sem); + lockevent_inc(rwsem_opt_wlock); + return true; + } + } + return false; +} + +static inline bool owner_on_cpu(struct task_struct *owner) +{ + /* + * As lock holder preemption issue, we both skip spinning if + * task is not on cpu or its cpu is preempted + */ + return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner)); +} + +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, + unsigned long nonspinnable) +{ + struct task_struct *owner; + unsigned long flags; + bool ret = true; + + if (need_resched()) { + lockevent_inc(rwsem_opt_fail); + return false; + } + + preempt_disable(); + rcu_read_lock(); + owner = rwsem_owner_flags(sem, &flags); + /* + * Don't check the read-owner as the entry may be stale. + */ + if ((flags & nonspinnable) || + (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner))) + ret = false; + rcu_read_unlock(); + preempt_enable(); + + lockevent_cond_inc(rwsem_opt_fail, !ret); + return ret; +} + +/* + * The rwsem_spin_on_owner() function returns the folowing 4 values + * depending on the lock owner state. + * OWNER_NULL : owner is currently NULL + * OWNER_WRITER: when owner changes and is a writer + * OWNER_READER: when owner changes and the new owner may be a reader. + * OWNER_NONSPINNABLE: + * when optimistic spinning has to stop because either the + * owner stops running, is unknown, or its timeslice has + * been used up. + */ +enum owner_state { + OWNER_NULL = 1 << 0, + OWNER_WRITER = 1 << 1, + OWNER_READER = 1 << 2, + OWNER_NONSPINNABLE = 1 << 3, +}; +#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER) + +static inline enum owner_state +rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable) +{ + if (flags & nonspinnable) + return OWNER_NONSPINNABLE; + + if (flags & RWSEM_READER_OWNED) + return OWNER_READER; + + return owner ? OWNER_WRITER : OWNER_NULL; +} + +static noinline enum owner_state +rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) +{ + struct task_struct *new, *owner; + unsigned long flags, new_flags; + enum owner_state state; + + owner = rwsem_owner_flags(sem, &flags); + state = rwsem_owner_state(owner, flags, nonspinnable); + if (state != OWNER_WRITER) + return state; + + rcu_read_lock(); + for (;;) { + /* + * When a waiting writer set the handoff flag, it may spin + * on the owner as well. Once that writer acquires the lock, + * we can spin on it. So we don't need to quit even when the + * handoff bit is set. + */ + new = rwsem_owner_flags(sem, &new_flags); + if ((new != owner) || (new_flags != flags)) { + state = rwsem_owner_state(new, new_flags, nonspinnable); + break; + } + + /* + * Ensure we emit the owner->on_cpu, dereference _after_ + * checking sem->owner still matches owner, if that fails, + * owner might point to free()d memory, if it still matches, + * the rcu_read_lock() ensures the memory stays valid. + */ + barrier(); + + if (need_resched() || !owner_on_cpu(owner)) { + state = OWNER_NONSPINNABLE; + break; + } + + cpu_relax(); + } + rcu_read_unlock(); + + return state; +} + +/* + * Calculate reader-owned rwsem spinning threshold for writer + * + * The more readers own the rwsem, the longer it will take for them to + * wind down and free the rwsem. So the empirical formula used to + * determine the actual spinning time limit here is: + * + * Spinning threshold = (10 + nr_readers/2)us + * + * The limit is capped to a maximum of 25us (30 readers). This is just + * a heuristic and is subjected to change in the future. + */ +static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem) +{ + long count = atomic_long_read(&sem->count); + int readers = count >> RWSEM_READER_SHIFT; + u64 delta; + + if (readers > 30) + readers = 30; + delta = (20 + readers) * NSEC_PER_USEC / 2; + + return sched_clock() + delta; +} + +static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock) +{ + bool taken = false; + int prev_owner_state = OWNER_NULL; + int loop = 0; + u64 rspin_threshold = 0; + unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE + : RWSEM_RD_NONSPINNABLE; + + preempt_disable(); + + /* sem->wait_lock should not be held when doing optimistic spinning */ + if (!osq_lock(&sem->osq)) + goto done; + + /* + * Optimistically spin on the owner field and attempt to acquire the + * lock whenever the owner changes. Spinning will be stopped when: + * 1) the owning writer isn't running; or + * 2) readers own the lock and spinning time has exceeded limit. + */ + for (;;) { + enum owner_state owner_state; + + owner_state = rwsem_spin_on_owner(sem, nonspinnable); + if (!(owner_state & OWNER_SPINNABLE)) + break; + + /* + * Try to acquire the lock + */ + taken = wlock ? rwsem_try_write_lock_unqueued(sem) + : rwsem_try_read_lock_unqueued(sem); + + if (taken) + break; + + /* + * Time-based reader-owned rwsem optimistic spinning + */ + if (wlock && (owner_state == OWNER_READER)) { + /* + * Re-initialize rspin_threshold every time when + * the owner state changes from non-reader to reader. + * This allows a writer to steal the lock in between + * 2 reader phases and have the threshold reset at + * the beginning of the 2nd reader phase. + */ + if (prev_owner_state != OWNER_READER) { + if (rwsem_test_oflags(sem, nonspinnable)) + break; + rspin_threshold = rwsem_rspin_threshold(sem); + loop = 0; + } + + /* + * Check time threshold once every 16 iterations to + * avoid calling sched_clock() too frequently so + * as to reduce the average latency between the times + * when the lock becomes free and when the spinner + * is ready to do a trylock. + */ + else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) { + rwsem_set_nonspinnable(sem); + lockevent_inc(rwsem_opt_nospin); + break; + } + } + + /* + * An RT task cannot do optimistic spinning if it cannot + * be sure the lock holder is running or live-lock may + * happen if the current task and the lock holder happen + * to run in the same CPU. However, aborting optimistic + * spinning while a NULL owner is detected may miss some + * opportunity where spinning can continue without causing + * problem. + * + * There are 2 possible cases where an RT task may be able + * to continue spinning. + * + * 1) The lock owner is in the process of releasing the + * lock, sem->owner is cleared but the lock has not + * been released yet. + * 2) The lock was free and owner cleared, but another + * task just comes in and acquire the lock before + * we try to get it. The new owner may be a spinnable + * writer. + * + * To take advantage of two scenarios listed agove, the RT + * task is made to retry one more time to see if it can + * acquire the lock or continue spinning on the new owning + * writer. Of course, if the time lag is long enough or the + * new owner is not a writer or spinnable, the RT task will + * quit spinning. + * + * If the owner is a writer, the need_resched() check is + * done inside rwsem_spin_on_owner(). If the owner is not + * a writer, need_resched() check needs to be done here. + */ + if (owner_state != OWNER_WRITER) { + if (need_resched()) + break; + if (rt_task(current) && + (prev_owner_state != OWNER_WRITER)) + break; + } + prev_owner_state = owner_state; + + /* + * The cpu_relax() call is a compiler barrier which forces + * everything in this loop to be re-loaded. We don't need + * memory barriers as we'll eventually observe the right + * values at the cost of a few extra spins. + */ + cpu_relax(); + } + osq_unlock(&sem->osq); +done: + preempt_enable(); + lockevent_cond_inc(rwsem_opt_fail, !taken); + return taken; +} + +/* + * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should + * only be called when the reader count reaches 0. + * + * This give writers better chance to acquire the rwsem first before + * readers when the rwsem was being held by readers for a relatively long + * period of time. Race can happen that an optimistic spinner may have + * just stolen the rwsem and set the owner, but just clearing the + * RWSEM_WR_NONSPINNABLE bit will do no harm anyway. + */ +static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) +{ + if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE)) + atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner); +} + +/* + * This function is called when the reader fails to acquire the lock via + * optimistic spinning. In this case we will still attempt to do a trylock + * when comparing the rwsem state right now with the state when entering + * the slowpath indicates that the reader is still in a valid reader phase. + * This happens when the following conditions are true: + * + * 1) The lock is currently reader owned, and + * 2) The lock is previously not reader-owned or the last read owner changes. + * + * In the former case, we have transitioned from a writer phase to a + * reader-phase while spinning. In the latter case, it means the reader + * phase hasn't ended when we entered the optimistic spinning loop. In + * both cases, the reader is eligible to acquire the lock. This is the + * secondary path where a read lock is acquired optimistically. + * + * The reader non-spinnable bit wasn't set at time of entry or it will + * not be here at all. + */ +static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem, + unsigned long last_rowner) +{ + unsigned long owner = atomic_long_read(&sem->owner); + + if (!(owner & RWSEM_READER_OWNED)) + return false; + + if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) && + rwsem_try_read_lock_unqueued(sem)) { + lockevent_inc(rwsem_opt_rlock2); + lockevent_add(rwsem_opt_fail, -1); + return true; + } + return false; +} +#else +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, + unsigned long nonspinnable) +{ + return false; +} + +static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock) +{ + return false; +} + +static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { } + +static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem, + unsigned long last_rowner) +{ + return false; +} + +static inline int +rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) +{ + return 0; +} +#define OWNER_NULL 1 +#endif + +/* + * Wait for the read lock to be granted + */ +static struct rw_semaphore __sched * +rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) +{ + long count, adjustment = -RWSEM_READER_BIAS; + struct rwsem_waiter waiter; + DEFINE_WAKE_Q(wake_q); + bool wake = false; + + /* + * Save the current read-owner of rwsem, if available, and the + * reader nonspinnable bit. + */ + waiter.last_rowner = atomic_long_read(&sem->owner); + if (!(waiter.last_rowner & RWSEM_READER_OWNED)) + waiter.last_rowner &= RWSEM_RD_NONSPINNABLE; + + if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE)) + goto queue; + + /* + * Undo read bias from down_read() and do optimistic spinning. + */ + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); + adjustment = 0; + if (rwsem_optimistic_spin(sem, false)) { + /* rwsem_optimistic_spin() implies ACQUIRE on success */ + /* + * Wake up other readers in the wait list if the front + * waiter is a reader. + */ + if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) { + raw_spin_lock_irq(&sem->wait_lock); + if (!list_empty(&sem->wait_list)) + rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, + &wake_q); + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + } + return sem; + } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) { + /* rwsem_reader_phase_trylock() implies ACQUIRE on success */ + return sem; + } + +queue: + waiter.task = current; + waiter.type = RWSEM_WAITING_FOR_READ; + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; + + raw_spin_lock_irq(&sem->wait_lock); + if (list_empty(&sem->wait_list)) { + /* + * In case the wait queue is empty and the lock isn't owned + * by a writer or has the handoff bit set, this reader can + * exit the slowpath and return immediately as its + * RWSEM_READER_BIAS has already been set in the count. + */ + if (adjustment && !(atomic_long_read(&sem->count) & + (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) { + /* Provide lock ACQUIRE */ + smp_acquire__after_ctrl_dep(); + raw_spin_unlock_irq(&sem->wait_lock); + rwsem_set_reader_owned(sem); + lockevent_inc(rwsem_rlock_fast); + return sem; + } + adjustment += RWSEM_FLAG_WAITERS; + } + list_add_tail(&waiter.list, &sem->wait_list); + + /* we're now waiting on the lock, but no longer actively locking */ + if (adjustment) + count = atomic_long_add_return(adjustment, &sem->count); + else + count = atomic_long_read(&sem->count); + + /* + * If there are no active locks, wake the front queued process(es). + * + * If there are no writers and we are first in the queue, + * wake our own waiter to join the existing active readers ! + */ + if (!(count & RWSEM_LOCK_MASK)) { + clear_wr_nonspinnable(sem); + wake = true; + } + if (wake || (!(count & RWSEM_WRITER_MASK) && + (adjustment & RWSEM_FLAG_WAITERS))) + rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); + + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + + /* wait to be given the lock */ + for (;;) { + set_current_state(state); + if (!smp_load_acquire(&waiter.task)) { + /* Matches rwsem_mark_wake()'s smp_store_release(). */ + break; + } + if (signal_pending_state(state, current)) { + raw_spin_lock_irq(&sem->wait_lock); + if (waiter.task) + goto out_nolock; + raw_spin_unlock_irq(&sem->wait_lock); + /* Ordered by sem->wait_lock against rwsem_mark_wake(). */ + break; + } + schedule(); + lockevent_inc(rwsem_sleep_reader); + } + + __set_current_state(TASK_RUNNING); + lockevent_inc(rwsem_rlock); + return sem; + +out_nolock: + list_del(&waiter.list); + if (list_empty(&sem->wait_list)) { + atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF, + &sem->count); + } + raw_spin_unlock_irq(&sem->wait_lock); + __set_current_state(TASK_RUNNING); + lockevent_inc(rwsem_rlock_fail); + return ERR_PTR(-EINTR); +} + +/* + * This function is called by the a write lock owner. So the owner value + * won't get changed by others. + */ +static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem, + bool disable) +{ + if (unlikely(disable)) { + atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner); + lockevent_inc(rwsem_opt_norspin); + } +} + +/* + * Wait until we successfully acquire the write lock + */ +static struct rw_semaphore * +rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) +{ + long count; + bool disable_rspin; + enum writer_wait_state wstate; + struct rwsem_waiter waiter; + struct rw_semaphore *ret = sem; + DEFINE_WAKE_Q(wake_q); + + /* do optimistic spinning and steal lock if possible */ + if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) && + rwsem_optimistic_spin(sem, true)) { + /* rwsem_optimistic_spin() implies ACQUIRE on success */ + return sem; + } + + /* + * Disable reader optimistic spinning for this rwsem after + * acquiring the write lock when the setting of the nonspinnable + * bits are observed. + */ + disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE; + + /* + * Optimistic spinning failed, proceed to the slowpath + * and block until we can acquire the sem. + */ + waiter.task = current; + waiter.type = RWSEM_WAITING_FOR_WRITE; + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; + + raw_spin_lock_irq(&sem->wait_lock); + + /* account for this before adding a new element to the list */ + wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST; + + list_add_tail(&waiter.list, &sem->wait_list); + + /* we're now waiting on the lock */ + if (wstate == WRITER_NOT_FIRST) { + count = atomic_long_read(&sem->count); + + /* + * If there were already threads queued before us and: + * 1) there are no active locks, wake the front + * queued process(es) as the handoff bit might be set. + * 2) there are no active writers and some readers, the lock + * must be read owned; so we try to wake any read lock + * waiters that were queued ahead of us. + */ + if (count & RWSEM_WRITER_MASK) + goto wait; + + rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) + ? RWSEM_WAKE_READERS + : RWSEM_WAKE_ANY, &wake_q); + + if (!wake_q_empty(&wake_q)) { + /* + * We want to minimize wait_lock hold time especially + * when a large number of readers are to be woken up. + */ + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + wake_q_init(&wake_q); /* Used again, reinit */ + raw_spin_lock_irq(&sem->wait_lock); + } + } else { + atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count); + } + +wait: + /* wait until we successfully acquire the lock */ + set_current_state(state); + for (;;) { + if (rwsem_try_write_lock(sem, wstate)) { + /* rwsem_try_write_lock() implies ACQUIRE on success */ + break; + } + + raw_spin_unlock_irq(&sem->wait_lock); + + /* + * After setting the handoff bit and failing to acquire + * the lock, attempt to spin on owner to accelerate lock + * transfer. If the previous owner is a on-cpu writer and it + * has just released the lock, OWNER_NULL will be returned. + * In this case, we attempt to acquire the lock again + * without sleeping. + */ + if (wstate == WRITER_HANDOFF && + rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL) + goto trylock_again; + + /* Block until there are no active lockers. */ + for (;;) { + if (signal_pending_state(state, current)) + goto out_nolock; + + schedule(); + lockevent_inc(rwsem_sleep_writer); + set_current_state(state); + /* + * If HANDOFF bit is set, unconditionally do + * a trylock. + */ + if (wstate == WRITER_HANDOFF) + break; + + if ((wstate == WRITER_NOT_FIRST) && + (rwsem_first_waiter(sem) == &waiter)) + wstate = WRITER_FIRST; + + count = atomic_long_read(&sem->count); + if (!(count & RWSEM_LOCK_MASK)) + break; + + /* + * The setting of the handoff bit is deferred + * until rwsem_try_write_lock() is called. + */ + if ((wstate == WRITER_FIRST) && (rt_task(current) || + time_after(jiffies, waiter.timeout))) { + wstate = WRITER_HANDOFF; + lockevent_inc(rwsem_wlock_handoff); + break; + } + } +trylock_again: + raw_spin_lock_irq(&sem->wait_lock); + } + __set_current_state(TASK_RUNNING); + list_del(&waiter.list); + rwsem_disable_reader_optspin(sem, disable_rspin); + raw_spin_unlock_irq(&sem->wait_lock); + lockevent_inc(rwsem_wlock); + + return ret; + +out_nolock: + __set_current_state(TASK_RUNNING); + raw_spin_lock_irq(&sem->wait_lock); + list_del(&waiter.list); + + if (unlikely(wstate == WRITER_HANDOFF)) + atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count); + + if (list_empty(&sem->wait_list)) + atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count); + else + rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + lockevent_inc(rwsem_wlock_fail); + + return ERR_PTR(-EINTR); +} + +/* + * handle waking up a waiter on the semaphore + * - up_read/up_write has decremented the active part of count if we come here + */ +static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count) +{ + unsigned long flags; + DEFINE_WAKE_Q(wake_q); + + raw_spin_lock_irqsave(&sem->wait_lock, flags); + + if (!list_empty(&sem->wait_list)) + rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); + + raw_spin_unlock_irqrestore(&sem->wait_lock, flags); + wake_up_q(&wake_q); + + return sem; +} + +/* + * downgrade a write lock into a read lock + * - caller incremented waiting part of count and discovered it still negative + * - just wake up any readers at the front of the queue + */ +static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) +{ + unsigned long flags; + DEFINE_WAKE_Q(wake_q); + + raw_spin_lock_irqsave(&sem->wait_lock, flags); + + if (!list_empty(&sem->wait_list)) + rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q); + + raw_spin_unlock_irqrestore(&sem->wait_lock, flags); + wake_up_q(&wake_q); + + return sem; +} + +/* + * lock for reading + */ +static inline void __down_read(struct rw_semaphore *sem) +{ + if (!rwsem_read_trylock(sem)) { + rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE); + DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); + } else { + rwsem_set_reader_owned(sem); + } +} + +static inline int __down_read_interruptible(struct rw_semaphore *sem) +{ + if (!rwsem_read_trylock(sem)) { + if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_INTERRUPTIBLE))) + return -EINTR; + DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); + } else { + rwsem_set_reader_owned(sem); + } + return 0; +} + +static inline int __down_read_killable(struct rw_semaphore *sem) +{ + if (!rwsem_read_trylock(sem)) { + if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE))) + return -EINTR; + DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); + } else { + rwsem_set_reader_owned(sem); + } + return 0; +} + +static inline int __down_read_trylock(struct rw_semaphore *sem) +{ + long tmp; + + DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem); + + /* + * Optimize for the case when the rwsem is not locked at all. + */ + tmp = RWSEM_UNLOCKED_VALUE; + do { + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + tmp + RWSEM_READER_BIAS)) { + rwsem_set_reader_owned(sem); + return 1; + } + } while (!(tmp & RWSEM_READ_FAILED_MASK)); + return 0; +} + +/* + * lock for writing + */ +static inline void __down_write(struct rw_semaphore *sem) +{ + long tmp = RWSEM_UNLOCKED_VALUE; + + if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + RWSEM_WRITER_LOCKED))) + rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE); + else + rwsem_set_owner(sem); +} + +static inline int __down_write_killable(struct rw_semaphore *sem) +{ + long tmp = RWSEM_UNLOCKED_VALUE; + + if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + RWSEM_WRITER_LOCKED))) { + if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE))) + return -EINTR; + } else { + rwsem_set_owner(sem); + } + return 0; +} + +static inline int __down_write_trylock(struct rw_semaphore *sem) +{ + long tmp; + + DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem); + + tmp = RWSEM_UNLOCKED_VALUE; + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + RWSEM_WRITER_LOCKED)) { + rwsem_set_owner(sem); + return true; + } + return false; +} + +/* + * unlock after reading + */ +static inline void __up_read(struct rw_semaphore *sem) +{ + long tmp; + + DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem); + DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); + + rwsem_clear_reader_owned(sem); + tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count); + DEBUG_RWSEMS_WARN_ON(tmp < 0, sem); + if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) == + RWSEM_FLAG_WAITERS)) { + clear_wr_nonspinnable(sem); + rwsem_wake(sem, tmp); + } +} + +/* + * unlock after writing + */ +static inline void __up_write(struct rw_semaphore *sem) +{ + long tmp; + + DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem); + /* + * sem->owner may differ from current if the ownership is transferred + * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits. + */ + DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && + !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem); + + rwsem_clear_owner(sem); + tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count); + if (unlikely(tmp & RWSEM_FLAG_WAITERS)) + rwsem_wake(sem, tmp); +} + +/* + * downgrade write lock to read lock + */ +static inline void __downgrade_write(struct rw_semaphore *sem) +{ + long tmp; + + /* + * When downgrading from exclusive to shared ownership, + * anything inside the write-locked region cannot leak + * into the read side. In contrast, anything in the + * read-locked region is ok to be re-ordered into the + * write side. As such, rely on RELEASE semantics. + */ + DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem); + tmp = atomic_long_fetch_add_release( + -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count); + rwsem_set_reader_owned(sem); + if (tmp & RWSEM_FLAG_WAITERS) + rwsem_downgrade_wake(sem); +} + +/* + * lock for reading + */ +void __sched down_read(struct rw_semaphore *sem) +{ + might_sleep(); + rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); + + LOCK_CONTENDED(sem, __down_read_trylock, __down_read); +} +EXPORT_SYMBOL(down_read); + +int __sched down_read_interruptible(struct rw_semaphore *sem) +{ + might_sleep(); + rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); + + if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) { + rwsem_release(&sem->dep_map, _RET_IP_); + return -EINTR; + } + + return 0; +} +EXPORT_SYMBOL(down_read_interruptible); + +int __sched down_read_killable(struct rw_semaphore *sem) +{ + might_sleep(); + rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); + + if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) { + rwsem_release(&sem->dep_map, _RET_IP_); + return -EINTR; + } + + return 0; +} +EXPORT_SYMBOL(down_read_killable); + +/* + * trylock for reading -- returns 1 if successful, 0 if contention + */ +int down_read_trylock(struct rw_semaphore *sem) +{ + int ret = __down_read_trylock(sem); + + if (ret == 1) + rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_); + return ret; +} +EXPORT_SYMBOL(down_read_trylock); + +/* + * lock for writing + */ +void __sched down_write(struct rw_semaphore *sem) +{ + might_sleep(); + rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); + LOCK_CONTENDED(sem, __down_write_trylock, __down_write); +} +EXPORT_SYMBOL(down_write); + +/* + * lock for writing + */ +int __sched down_write_killable(struct rw_semaphore *sem) +{ + might_sleep(); + rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); + + if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock, + __down_write_killable)) { + rwsem_release(&sem->dep_map, _RET_IP_); + return -EINTR; + } + + return 0; +} +EXPORT_SYMBOL(down_write_killable); + +/* + * trylock for writing -- returns 1 if successful, 0 if contention + */ +int down_write_trylock(struct rw_semaphore *sem) +{ + int ret = __down_write_trylock(sem); + + if (ret == 1) + rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_); + + return ret; +} +EXPORT_SYMBOL(down_write_trylock); + +/* + * release a read lock + */ +void up_read(struct rw_semaphore *sem) +{ + rwsem_release(&sem->dep_map, _RET_IP_); + __up_read(sem); +} +EXPORT_SYMBOL(up_read); + +/* + * release a write lock + */ +void up_write(struct rw_semaphore *sem) +{ + rwsem_release(&sem->dep_map, _RET_IP_); + __up_write(sem); +} +EXPORT_SYMBOL(up_write); + +/* + * downgrade write lock to read lock + */ +void downgrade_write(struct rw_semaphore *sem) +{ + lock_downgrade(&sem->dep_map, _RET_IP_); + __downgrade_write(sem); +} +EXPORT_SYMBOL(downgrade_write); + +#ifdef CONFIG_DEBUG_LOCK_ALLOC + +void down_read_nested(struct rw_semaphore *sem, int subclass) +{ + might_sleep(); + rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_); + LOCK_CONTENDED(sem, __down_read_trylock, __down_read); +} +EXPORT_SYMBOL(down_read_nested); + +int down_read_killable_nested(struct rw_semaphore *sem, int subclass) +{ + might_sleep(); + rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_); + + if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) { + rwsem_release(&sem->dep_map, _RET_IP_); + return -EINTR; + } + + return 0; +} +EXPORT_SYMBOL(down_read_killable_nested); + +void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest) +{ + might_sleep(); + rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_); + LOCK_CONTENDED(sem, __down_write_trylock, __down_write); +} +EXPORT_SYMBOL(_down_write_nest_lock); + +void down_read_non_owner(struct rw_semaphore *sem) +{ + might_sleep(); + __down_read(sem); + __rwsem_set_reader_owned(sem, NULL); +} +EXPORT_SYMBOL(down_read_non_owner); + +void down_write_nested(struct rw_semaphore *sem, int subclass) +{ + might_sleep(); + rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_); + LOCK_CONTENDED(sem, __down_write_trylock, __down_write); +} +EXPORT_SYMBOL(down_write_nested); + +int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass) +{ + might_sleep(); + rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_); + + if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock, + __down_write_killable)) { + rwsem_release(&sem->dep_map, _RET_IP_); + return -EINTR; + } + + return 0; +} +EXPORT_SYMBOL(down_write_killable_nested); + +void up_read_non_owner(struct rw_semaphore *sem) +{ + DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); + __up_read(sem); +} +EXPORT_SYMBOL(up_read_non_owner); + +#endif |