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-rw-r--r--debian/patches-rt/0173-locking-rtmutex-add-rwsem-implementation-based-on-rt.patch455
1 files changed, 455 insertions, 0 deletions
diff --git a/debian/patches-rt/0173-locking-rtmutex-add-rwsem-implementation-based-on-rt.patch b/debian/patches-rt/0173-locking-rtmutex-add-rwsem-implementation-based-on-rt.patch
new file mode 100644
index 000000000..d1595ab45
--- /dev/null
+++ b/debian/patches-rt/0173-locking-rtmutex-add-rwsem-implementation-based-on-rt.patch
@@ -0,0 +1,455 @@
+From 9095a37abced402fb5197fef8d126069f63ebfb4 Mon Sep 17 00:00:00 2001
+From: Thomas Gleixner <tglx@linutronix.de>
+Date: Thu, 12 Oct 2017 17:28:34 +0200
+Subject: [PATCH 173/323] locking/rtmutex: add rwsem implementation based on
+ rtmutex
+Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.204-rt100.tar.xz
+
+The RT specific R/W semaphore implementation restricts the number of readers
+to one because a writer cannot block on multiple readers and inherit its
+priority or budget.
+
+The single reader restricting is painful in various ways:
+
+ - Performance bottleneck for multi-threaded applications in the page fault
+ path (mmap sem)
+
+ - Progress blocker for drivers which are carefully crafted to avoid the
+ potential reader/writer deadlock in mainline.
+
+The analysis of the writer code paths shows, that properly written RT tasks
+should not take them. Syscalls like mmap(), file access which take mmap sem
+write locked have unbound latencies which are completely unrelated to mmap
+sem. Other R/W sem users like graphics drivers are not suitable for RT tasks
+either.
+
+So there is little risk to hurt RT tasks when the RT rwsem implementation is
+changed in the following way:
+
+ - Allow concurrent readers
+
+ - Make writers block until the last reader left the critical section. This
+ blocking is not subject to priority/budget inheritance.
+
+ - Readers blocked on a writer inherit their priority/budget in the normal
+ way.
+
+There is a drawback with this scheme. R/W semaphores become writer unfair
+though the applications which have triggered writer starvation (mostly on
+mmap_sem) in the past are not really the typical workloads running on a RT
+system. So while it's unlikely to hit writer starvation, it's possible. If
+there are unexpected workloads on RT systems triggering it, we need to rethink
+the approach.
+
+Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+---
+ include/linux/rwsem-rt.h | 70 +++++++++
+ kernel/locking/rwsem-rt.c | 318 ++++++++++++++++++++++++++++++++++++++
+ 2 files changed, 388 insertions(+)
+ create mode 100644 include/linux/rwsem-rt.h
+ create mode 100644 kernel/locking/rwsem-rt.c
+
+diff --git a/include/linux/rwsem-rt.h b/include/linux/rwsem-rt.h
+new file mode 100644
+index 000000000000..0ba8aae9a198
+--- /dev/null
++++ b/include/linux/rwsem-rt.h
+@@ -0,0 +1,70 @@
++// SPDX-License-Identifier: GPL-2.0-only
++#ifndef _LINUX_RWSEM_RT_H
++#define _LINUX_RWSEM_RT_H
++
++#ifndef _LINUX_RWSEM_H
++#error "Include rwsem.h"
++#endif
++
++#include <linux/rtmutex.h>
++#include <linux/swait.h>
++
++#define READER_BIAS (1U << 31)
++#define WRITER_BIAS (1U << 30)
++
++struct rw_semaphore {
++ atomic_t readers;
++ struct rt_mutex rtmutex;
++#ifdef CONFIG_DEBUG_LOCK_ALLOC
++ struct lockdep_map dep_map;
++#endif
++};
++
++#define __RWSEM_INITIALIZER(name) \
++{ \
++ .readers = ATOMIC_INIT(READER_BIAS), \
++ .rtmutex = __RT_MUTEX_INITIALIZER(name.rtmutex), \
++ RW_DEP_MAP_INIT(name) \
++}
++
++#define DECLARE_RWSEM(lockname) \
++ struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
++
++extern void __rwsem_init(struct rw_semaphore *rwsem, const char *name,
++ struct lock_class_key *key);
++
++#define __init_rwsem(sem, name, key) \
++do { \
++ rt_mutex_init(&(sem)->rtmutex); \
++ __rwsem_init((sem), (name), (key)); \
++} while (0)
++
++#define init_rwsem(sem) \
++do { \
++ static struct lock_class_key __key; \
++ \
++ __init_rwsem((sem), #sem, &__key); \
++} while (0)
++
++static inline int rwsem_is_locked(struct rw_semaphore *sem)
++{
++ return atomic_read(&sem->readers) != READER_BIAS;
++}
++
++static inline int rwsem_is_contended(struct rw_semaphore *sem)
++{
++ return atomic_read(&sem->readers) > 0;
++}
++
++extern void __down_read(struct rw_semaphore *sem);
++extern int __down_read_interruptible(struct rw_semaphore *sem);
++extern int __down_read_killable(struct rw_semaphore *sem);
++extern int __down_read_trylock(struct rw_semaphore *sem);
++extern void __down_write(struct rw_semaphore *sem);
++extern int __must_check __down_write_killable(struct rw_semaphore *sem);
++extern int __down_write_trylock(struct rw_semaphore *sem);
++extern void __up_read(struct rw_semaphore *sem);
++extern void __up_write(struct rw_semaphore *sem);
++extern void __downgrade_write(struct rw_semaphore *sem);
++
++#endif
+diff --git a/kernel/locking/rwsem-rt.c b/kernel/locking/rwsem-rt.c
+new file mode 100644
+index 000000000000..a0771c150041
+--- /dev/null
++++ b/kernel/locking/rwsem-rt.c
+@@ -0,0 +1,318 @@
++// SPDX-License-Identifier: GPL-2.0-only
++#include <linux/rwsem.h>
++#include <linux/sched/debug.h>
++#include <linux/sched/signal.h>
++#include <linux/export.h>
++#include <linux/blkdev.h>
++
++#include "rtmutex_common.h"
++
++/*
++ * RT-specific reader/writer semaphores
++ *
++ * down_write()
++ * 1) Lock sem->rtmutex
++ * 2) Remove the reader BIAS to force readers into the slow path
++ * 3) Wait until all readers have left the critical region
++ * 4) Mark it write locked
++ *
++ * up_write()
++ * 1) Remove the write locked marker
++ * 2) Set the reader BIAS so readers can use the fast path again
++ * 3) Unlock sem->rtmutex to release blocked readers
++ *
++ * down_read()
++ * 1) Try fast path acquisition (reader BIAS is set)
++ * 2) Take sem->rtmutex.wait_lock which protects the writelocked flag
++ * 3) If !writelocked, acquire it for read
++ * 4) If writelocked, block on sem->rtmutex
++ * 5) unlock sem->rtmutex, goto 1)
++ *
++ * up_read()
++ * 1) Try fast path release (reader count != 1)
++ * 2) Wake the writer waiting in down_write()#3
++ *
++ * down_read()#3 has the consequence, that rw semaphores on RT are not writer
++ * fair, but writers, which should be avoided in RT tasks (think mmap_sem),
++ * are subject to the rtmutex priority/DL inheritance mechanism.
++ *
++ * It's possible to make the rw semaphores writer fair by keeping a list of
++ * active readers. A blocked writer would force all newly incoming readers to
++ * block on the rtmutex, but the rtmutex would have to be proxy locked for one
++ * reader after the other. We can't use multi-reader inheritance because there
++ * is no way to support that with SCHED_DEADLINE. Implementing the one by one
++ * reader boosting/handover mechanism is a major surgery for a very dubious
++ * value.
++ *
++ * The risk of writer starvation is there, but the pathological use cases
++ * which trigger it are not necessarily the typical RT workloads.
++ */
++
++void __rwsem_init(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(&sem->dep_map, name, key, 0);
++#endif
++ atomic_set(&sem->readers, READER_BIAS);
++}
++EXPORT_SYMBOL(__rwsem_init);
++
++int __down_read_trylock(struct rw_semaphore *sem)
++{
++ int r, old;
++
++ /*
++ * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
++ * set.
++ */
++ for (r = atomic_read(&sem->readers); r < 0;) {
++ old = atomic_cmpxchg(&sem->readers, r, r + 1);
++ if (likely(old == r))
++ return 1;
++ r = old;
++ }
++ return 0;
++}
++
++static int __sched __down_read_common(struct rw_semaphore *sem, int state)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ struct rt_mutex_waiter waiter;
++ int ret;
++
++ if (__down_read_trylock(sem))
++ return 0;
++
++ /*
++ * Flush blk before ->pi_blocked_on is set. At schedule() time it is too
++ * late if one of the callbacks needs to acquire a sleeping lock.
++ */
++ if (blk_needs_flush_plug(current))
++ blk_schedule_flush_plug(current);
++
++ might_sleep();
++ raw_spin_lock_irq(&m->wait_lock);
++ /*
++ * Allow readers as long as the writer has not completely
++ * acquired the semaphore for write.
++ */
++ if (atomic_read(&sem->readers) != WRITER_BIAS) {
++ atomic_inc(&sem->readers);
++ raw_spin_unlock_irq(&m->wait_lock);
++ return 0;
++ }
++
++ /*
++ * Call into the slow lock path with the rtmutex->wait_lock
++ * held, so this can't result in the following race:
++ *
++ * Reader1 Reader2 Writer
++ * down_read()
++ * down_write()
++ * rtmutex_lock(m)
++ * swait()
++ * down_read()
++ * unlock(m->wait_lock)
++ * up_read()
++ * swake()
++ * lock(m->wait_lock)
++ * sem->writelocked=true
++ * unlock(m->wait_lock)
++ *
++ * up_write()
++ * sem->writelocked=false
++ * rtmutex_unlock(m)
++ * down_read()
++ * down_write()
++ * rtmutex_lock(m)
++ * swait()
++ * rtmutex_lock(m)
++ *
++ * That would put Reader1 behind the writer waiting on
++ * Reader2 to call up_read() which might be unbound.
++ */
++ rt_mutex_init_waiter(&waiter, false);
++ ret = rt_mutex_slowlock_locked(m, state, NULL, RT_MUTEX_MIN_CHAINWALK,
++ &waiter);
++ /*
++ * The slowlock() above is guaranteed to return with the rtmutex (for
++ * ret = 0) is now held, so there can't be a writer active. Increment
++ * the reader count and immediately drop the rtmutex again.
++ * For ret != 0 we don't hold the rtmutex and need unlock the wait_lock.
++ * We don't own the lock then.
++ */
++ if (!ret)
++ atomic_inc(&sem->readers);
++ raw_spin_unlock_irq(&m->wait_lock);
++ if (!ret)
++ __rt_mutex_unlock(m);
++
++ debug_rt_mutex_free_waiter(&waiter);
++ return ret;
++}
++
++void __down_read(struct rw_semaphore *sem)
++{
++ int ret;
++
++ ret = __down_read_common(sem, TASK_UNINTERRUPTIBLE);
++ WARN_ON_ONCE(ret);
++}
++
++int __down_read_interruptible(struct rw_semaphore *sem)
++{
++ int ret;
++
++ ret = __down_read_common(sem, TASK_INTERRUPTIBLE);
++ if (likely(!ret))
++ return ret;
++ WARN_ONCE(ret != -EINTR, "Unexpected state: %d\n", ret);
++ return -EINTR;
++}
++
++int __down_read_killable(struct rw_semaphore *sem)
++{
++ int ret;
++
++ ret = __down_read_common(sem, TASK_KILLABLE);
++ if (likely(!ret))
++ return ret;
++ WARN_ONCE(ret != -EINTR, "Unexpected state: %d\n", ret);
++ return -EINTR;
++}
++
++void __up_read(struct rw_semaphore *sem)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ struct task_struct *tsk;
++
++ /*
++ * sem->readers can only hit 0 when a writer is waiting for the
++ * active readers to leave the critical region.
++ */
++ if (!atomic_dec_and_test(&sem->readers))
++ return;
++
++ might_sleep();
++ raw_spin_lock_irq(&m->wait_lock);
++ /*
++ * Wake the writer, i.e. the rtmutex owner. It might release the
++ * rtmutex concurrently in the fast path (due to a signal), but to
++ * clean up the rwsem it needs to acquire m->wait_lock. The worst
++ * case which can happen is a spurious wakeup.
++ */
++ tsk = rt_mutex_owner(m);
++ if (tsk)
++ wake_up_process(tsk);
++
++ raw_spin_unlock_irq(&m->wait_lock);
++}
++
++static void __up_write_unlock(struct rw_semaphore *sem, int bias,
++ unsigned long flags)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++
++ atomic_add(READER_BIAS - bias, &sem->readers);
++ raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++ __rt_mutex_unlock(m);
++}
++
++static int __sched __down_write_common(struct rw_semaphore *sem, int state)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ unsigned long flags;
++
++ /*
++ * Flush blk before ->pi_blocked_on is set. At schedule() time it is too
++ * late if one of the callbacks needs to acquire a sleeping lock.
++ */
++ if (blk_needs_flush_plug(current))
++ blk_schedule_flush_plug(current);
++
++ /* Take the rtmutex as a first step */
++ if (__rt_mutex_lock_state(m, state))
++ return -EINTR;
++
++ /* Force readers into slow path */
++ atomic_sub(READER_BIAS, &sem->readers);
++ might_sleep();
++
++ set_current_state(state);
++ for (;;) {
++ raw_spin_lock_irqsave(&m->wait_lock, flags);
++ /* Have all readers left the critical region? */
++ if (!atomic_read(&sem->readers)) {
++ atomic_set(&sem->readers, WRITER_BIAS);
++ __set_current_state(TASK_RUNNING);
++ raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++ return 0;
++ }
++
++ if (signal_pending_state(state, current)) {
++ __set_current_state(TASK_RUNNING);
++ __up_write_unlock(sem, 0, flags);
++ return -EINTR;
++ }
++ raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++
++ if (atomic_read(&sem->readers) != 0) {
++ schedule();
++ set_current_state(state);
++ }
++ }
++}
++
++void __sched __down_write(struct rw_semaphore *sem)
++{
++ __down_write_common(sem, TASK_UNINTERRUPTIBLE);
++}
++
++int __sched __down_write_killable(struct rw_semaphore *sem)
++{
++ return __down_write_common(sem, TASK_KILLABLE);
++}
++
++int __down_write_trylock(struct rw_semaphore *sem)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ unsigned long flags;
++
++ if (!__rt_mutex_trylock(m))
++ return 0;
++
++ atomic_sub(READER_BIAS, &sem->readers);
++
++ raw_spin_lock_irqsave(&m->wait_lock, flags);
++ if (!atomic_read(&sem->readers)) {
++ atomic_set(&sem->readers, WRITER_BIAS);
++ raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++ return 1;
++ }
++ __up_write_unlock(sem, 0, flags);
++ return 0;
++}
++
++void __up_write(struct rw_semaphore *sem)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ unsigned long flags;
++
++ raw_spin_lock_irqsave(&m->wait_lock, flags);
++ __up_write_unlock(sem, WRITER_BIAS, flags);
++}
++
++void __downgrade_write(struct rw_semaphore *sem)
++{
++ struct rt_mutex *m = &sem->rtmutex;
++ unsigned long flags;
++
++ raw_spin_lock_irqsave(&m->wait_lock, flags);
++ /* Release it and account current as reader */
++ __up_write_unlock(sem, WRITER_BIAS - 1, flags);
++}
+--
+2.43.0
+