1 files changed, 430 insertions, 0 deletions
diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
new file mode 100644
index 000000000..3be428c16
--- /dev/null
+++ b/drivers/tty/tty_ldsem.c
@@ -0,0 +1,430 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Ldisc rw semaphore
+ *
+ * The ldisc semaphore is semantically a rw_semaphore but which enforces
+ * an alternate policy, namely:
+ *   1) Supports lock wait timeouts
+ *   2) Write waiter has priority
+ *   3) Downgrading is not supported
+ *
+ * Implementation notes:
+ *   1) Upper half of semaphore count is a wait count (differs from rwsem
+ *	in that rwsem normalizes the upper half to the wait bias)
+ *   2) Lacks overflow checking
+ *
+ * The generic counting was copied and modified from include/asm-generic/rwsem.h
+ * by Paul Mackerras <paulus@samba.org>.
+ *
+ * The scheduling policy was copied and modified from lib/rwsem.c
+ * Written by David Howells (dhowells@redhat.com).
+ *
+ * This implementation incorporates the write lock stealing work of
+ * Michel Lespinasse <walken@google.com>.
+ *
+ * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com>
+ */
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/atomic.h>
+#include <linux/tty.h>
+#include <linux/sched.h>
+#include <linux/sched/debug.h>
+#include <linux/sched/task.h>
+
+
+#if BITS_PER_LONG == 64
+# define LDSEM_ACTIVE_MASK	0xffffffffL
+#else
+# define LDSEM_ACTIVE_MASK	0x0000ffffL
+#endif
+
+#define LDSEM_UNLOCKED		0L
+#define LDSEM_ACTIVE_BIAS	1L
+#define LDSEM_WAIT_BIAS		(-LDSEM_ACTIVE_MASK-1)
+#define LDSEM_READ_BIAS		LDSEM_ACTIVE_BIAS
+#define LDSEM_WRITE_BIAS	(LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
+
+struct ldsem_waiter {
+	struct list_head list;
+	struct task_struct *task;
+};
+
+/*
+ * Initialize an ldsem:
+ */
+void __init_ldsem(struct ld_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(&sem->dep_map, name, key, 0);
+#endif
+	atomic_long_set(&sem->count, LDSEM_UNLOCKED);
+	sem->wait_readers = 0;
+	raw_spin_lock_init(&sem->wait_lock);
+	INIT_LIST_HEAD(&sem->read_wait);
+	INIT_LIST_HEAD(&sem->write_wait);
+}
+
+static void __ldsem_wake_readers(struct ld_semaphore *sem)
+{
+	struct ldsem_waiter *waiter, *next;
+	struct task_struct *tsk;
+	long adjust, count;
+
+	/*
+	 * Try to grant read locks to all readers on the read wait list.
+	 * Note the 'active part' of the count is incremented by
+	 * the number of readers before waking any processes up.
+	 */
+	adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
+	count = atomic_long_add_return(adjust, &sem->count);
+	do {
+		if (count > 0)
+			break;
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust))
+			return;
+	} while (1);
+
+	list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
+		tsk = waiter->task;
+		smp_store_release(&waiter->task, NULL);
+		wake_up_process(tsk);
+		put_task_struct(tsk);
+	}
+	INIT_LIST_HEAD(&sem->read_wait);
+	sem->wait_readers = 0;
+}
+
+static inline int writer_trylock(struct ld_semaphore *sem)
+{
+	/*
+	 * Only wake this writer if the active part of the count can be
+	 * transitioned from 0 -> 1
+	 */
+	long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count);
+	do {
+		if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
+			return 1;
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS))
+			return 0;
+	} while (1);
+}
+
+static void __ldsem_wake_writer(struct ld_semaphore *sem)
+{
+	struct ldsem_waiter *waiter;
+
+	waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
+	wake_up_process(waiter->task);
+}
+
+/*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here from up_xxxx(), then:
+ *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
+ *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
+ * - the spinlock must be held by the caller
+ * - woken process blocks are discarded from the list after having task zeroed
+ */
+static void __ldsem_wake(struct ld_semaphore *sem)
+{
+	if (!list_empty(&sem->write_wait))
+		__ldsem_wake_writer(sem);
+	else if (!list_empty(&sem->read_wait))
+		__ldsem_wake_readers(sem);
+}
+
+static void ldsem_wake(struct ld_semaphore *sem)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&sem->wait_lock, flags);
+	__ldsem_wake(sem);
+	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
+/*
+ * wait for the read lock to be granted
+ */
+static struct ld_semaphore __sched *
+down_read_failed(struct ld_semaphore *sem, long count, long timeout)
+{
+	struct ldsem_waiter waiter;
+	long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
+
+	/* set up my own style of waitqueue */
+	raw_spin_lock_irq(&sem->wait_lock);
+
+	/*
+	 * Try to reverse the lock attempt but if the count has changed
+	 * so that reversing fails, check if there are no waiters,
+	 * and early-out if not
+	 */
+	do {
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) {
+			count += adjust;
+			break;
+		}
+		if (count > 0) {
+			raw_spin_unlock_irq(&sem->wait_lock);
+			return sem;
+		}
+	} while (1);
+
+	list_add_tail(&waiter.list, &sem->read_wait);
+	sem->wait_readers++;
+
+	waiter.task = current;
+	get_task_struct(current);
+
+	/* if there are no active locks, wake the new lock owner(s) */
+	if ((count & LDSEM_ACTIVE_MASK) == 0)
+		__ldsem_wake(sem);
+
+	raw_spin_unlock_irq(&sem->wait_lock);
+
+	/* wait to be given the lock */
+	for (;;) {
+		set_current_state(TASK_UNINTERRUPTIBLE);
+
+		if (!smp_load_acquire(&waiter.task))
+			break;
+		if (!timeout)
+			break;
+		timeout = schedule_timeout(timeout);
+	}
+
+	__set_current_state(TASK_RUNNING);
+
+	if (!timeout) {
+		/*
+		 * Lock timed out but check if this task was just
+		 * granted lock ownership - if so, pretend there
+		 * was no timeout; otherwise, cleanup lock wait.
+		 */
+		raw_spin_lock_irq(&sem->wait_lock);
+		if (waiter.task) {
+			atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
+			sem->wait_readers--;
+			list_del(&waiter.list);
+			raw_spin_unlock_irq(&sem->wait_lock);
+			put_task_struct(waiter.task);
+			return NULL;
+		}
+		raw_spin_unlock_irq(&sem->wait_lock);
+	}
+
+	return sem;
+}
+
+/*
+ * wait for the write lock to be granted
+ */
+static struct ld_semaphore __sched *
+down_write_failed(struct ld_semaphore *sem, long count, long timeout)
+{
+	struct ldsem_waiter waiter;
+	long adjust = -LDSEM_ACTIVE_BIAS;
+	int locked = 0;
+
+	/* set up my own style of waitqueue */
+	raw_spin_lock_irq(&sem->wait_lock);
+
+	/*
+	 * Try to reverse the lock attempt but if the count has changed
+	 * so that reversing fails, check if the lock is now owned,
+	 * and early-out if so.
+	 */
+	do {
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust))
+			break;
+		if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
+			raw_spin_unlock_irq(&sem->wait_lock);
+			return sem;
+		}
+	} while (1);
+
+	list_add_tail(&waiter.list, &sem->write_wait);
+
+	waiter.task = current;
+
+	set_current_state(TASK_UNINTERRUPTIBLE);
+	for (;;) {
+		if (!timeout)
+			break;
+		raw_spin_unlock_irq(&sem->wait_lock);
+		timeout = schedule_timeout(timeout);
+		raw_spin_lock_irq(&sem->wait_lock);
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		locked = writer_trylock(sem);
+		if (locked)
+			break;
+	}
+
+	if (!locked)
+		atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
+	list_del(&waiter.list);
+
+	/*
+	 * In case of timeout, wake up every reader who gave the right of way
+	 * to writer. Prevent separation readers into two groups:
+	 * one that helds semaphore and another that sleeps.
+	 * (in case of no contention with a writer)
+	 */
+	if (!locked && list_empty(&sem->write_wait))
+		__ldsem_wake_readers(sem);
+
+	raw_spin_unlock_irq(&sem->wait_lock);
+
+	__set_current_state(TASK_RUNNING);
+
+	/* lock wait may have timed out */
+	if (!locked)
+		return NULL;
+	return sem;
+}
+
+
+
+static int __ldsem_down_read_nested(struct ld_semaphore *sem,
+					   int subclass, long timeout)
+{
+	long count;
+
+	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
+
+	count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count);
+	if (count <= 0) {
+		lock_contended(&sem->dep_map, _RET_IP_);
+		if (!down_read_failed(sem, count, timeout)) {
+			rwsem_release(&sem->dep_map, _RET_IP_);
+			return 0;
+		}
+	}
+	lock_acquired(&sem->dep_map, _RET_IP_);
+	return 1;
+}
+
+static int __ldsem_down_write_nested(struct ld_semaphore *sem,
+					    int subclass, long timeout)
+{
+	long count;
+
+	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
+
+	count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count);
+	if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
+		lock_contended(&sem->dep_map, _RET_IP_);
+		if (!down_write_failed(sem, count, timeout)) {
+			rwsem_release(&sem->dep_map, _RET_IP_);
+			return 0;
+		}
+	}
+	lock_acquired(&sem->dep_map, _RET_IP_);
+	return 1;
+}
+
+
+/*
+ * lock for reading -- returns 1 if successful, 0 if timed out
+ */
+int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
+{
+	might_sleep();
+	return __ldsem_down_read_nested(sem, 0, timeout);
+}
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+int ldsem_down_read_trylock(struct ld_semaphore *sem)
+{
+	long count = atomic_long_read(&sem->count);
+
+	while (count >= 0) {
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) {
+			rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
+			lock_acquired(&sem->dep_map, _RET_IP_);
+			return 1;
+		}
+	}
+	return 0;
+}
+
+/*
+ * lock for writing -- returns 1 if successful, 0 if timed out
+ */
+int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
+{
+	might_sleep();
+	return __ldsem_down_write_nested(sem, 0, timeout);
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+int ldsem_down_write_trylock(struct ld_semaphore *sem)
+{
+	long count = atomic_long_read(&sem->count);
+
+	while ((count & LDSEM_ACTIVE_MASK) == 0) {
+		if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_WRITE_BIAS)) {
+			rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
+			lock_acquired(&sem->dep_map, _RET_IP_);
+			return 1;
+		}
+	}
+	return 0;
+}
+
+/*
+ * release a read lock
+ */
+void ldsem_up_read(struct ld_semaphore *sem)
+{
+	long count;
+
+	rwsem_release(&sem->dep_map, _RET_IP_);
+
+	count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);
+	if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
+		ldsem_wake(sem);
+}
+
+/*
+ * release a write lock
+ */
+void ldsem_up_write(struct ld_semaphore *sem)
+{
+	long count;
+
+	rwsem_release(&sem->dep_map, _RET_IP_);
+
+	count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);
+	if (count < 0)
+		ldsem_wake(sem);
+}
+
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+
+int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
+{
+	might_sleep();
+	return __ldsem_down_read_nested(sem, subclass, timeout);
+}
+
+int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
+			    long timeout)
+{
+	might_sleep();
+	return __ldsem_down_write_nested(sem, subclass, timeout);
+}
+
+#endif