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Commit 9d659ae1 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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locking/mutex: Add lock handoff to avoid starvation 



Implement lock handoff to avoid lock starvation.

Lock starvation is possible because mutex_lock() allows lock stealing,
where a running (or optimistic spinning) task beats the woken waiter
to the acquire.

Lock stealing is an important performance optimization because waiting
for a waiter to wake up and get runtime can take a significant time,
during which everyboy would stall on the lock.

The down-side is of course that it allows for starvation.

This patch has the waiter requesting a handoff if it fails to acquire
the lock upon waking. This re-introduces some of the wait time,
because once we do a handoff we have to wait for the waiter to wake up
again.

A future patch will add a round of optimistic spinning to attempt to
alleviate this penalty, but if that turns out to not be enough, we can
add a counter and only request handoff after multiple failed wakeups.

There are a few tricky implementation details:

 - accepting a handoff must only be done in the wait-loop. Since the
   handoff condition is owner == current, it can easily cause
   recursive locking trouble.

 - accepting the handoff must be careful to provide the ACQUIRE
   semantics.

 - having the HANDOFF bit set on unlock requires care, we must not
   clear the owner.

 - we must be careful to not leave HANDOFF set after we've acquired
   the lock. The tricky scenario is setting the HANDOFF bit on an
   unlocked mutex.

Tested-by: default avatarJason Low <jason.low2@hpe.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarWaiman Long <Waiman.Long@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a3ea3d9b

kernel/locking/mutex.c

+119 −23
Original line number Diff line number Diff line
@@ -54,8 +54,10 @@ EXPORT_SYMBOL(__mutex_init); 
 * bits to store extra state.

 *

 * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.

 * Bit1 indicates unlock needs to hand the lock to the top-waiter

 */

#define MUTEX_FLAG_WAITERS	0x01

#define MUTEX_FLAG_HANDOFF	0x02



#define MUTEX_FLAGS		0x03
@@ -71,20 +73,48 @@ static inline unsigned long __owner_flags(unsigned long owner) 


/*

 * Actual trylock that will work on any unlocked state.

 *

 * When setting the owner field, we must preserve the low flag bits.

 *

 * Be careful with @handoff, only set that in a wait-loop (where you set

 * HANDOFF) to avoid recursive lock attempts.

 */

static inline bool __mutex_trylock(struct mutex *lock)

static inline bool __mutex_trylock(struct mutex *lock, const bool handoff)

{

	unsigned long owner, curr = (unsigned long)current;



	owner = atomic_long_read(&lock->owner);

	for (;;) { /* must loop, can race against a flag */

		unsigned long old;

		unsigned long old, flags = __owner_flags(owner);



		if (__owner_task(owner)) {

			if (handoff && unlikely(__owner_task(owner) == current)) {

				/*

				 * Provide ACQUIRE semantics for the lock-handoff.

				 *

				 * We cannot easily use load-acquire here, since

				 * the actual load is a failed cmpxchg, which

				 * doesn't imply any barriers.

				 *

				 * Also, this is a fairly unlikely scenario, and

				 * this contains the cost.

				 */

				smp_mb(); /* ACQUIRE */

				return true;

			}



		if (__owner_task(owner))

			return false;

		}



		old = atomic_long_cmpxchg_acquire(&lock->owner, owner,

						  curr | __owner_flags(owner));

		/*

		 * We set the HANDOFF bit, we must make sure it doesn't live

		 * past the point where we acquire it. This would be possible

		 * if we (accidentally) set the bit on an unlocked mutex.

		 */

		if (handoff)

			flags &= ~MUTEX_FLAG_HANDOFF;



		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);

		if (old == owner)

			return true;
@@ -134,6 +164,39 @@ static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag) 
	atomic_long_andnot(flag, &lock->owner);

}



static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)

{

	return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;

}



/*

 * Give up ownership to a specific task, when @task = NULL, this is equivalent

 * to a regular unlock. Clears HANDOFF, preserves WAITERS. Provides RELEASE

 * semantics like a regular unlock, the __mutex_trylock() provides matching

 * ACQUIRE semantics for the handoff.

 */

static void __mutex_handoff(struct mutex *lock, struct task_struct *task)

{

	unsigned long owner = atomic_long_read(&lock->owner);



	for (;;) {

		unsigned long old, new;



#ifdef CONFIG_DEBUG_MUTEXES

		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);

#endif



		new = (owner & MUTEX_FLAG_WAITERS);

		new |= (unsigned long)task;



		old = atomic_long_cmpxchg_release(&lock->owner, owner, new);

		if (old == owner)

			break;



		owner = old;

	}

}



#ifndef CONFIG_DEBUG_LOCK_ALLOC

/*

 * We split the mutex lock/unlock logic into separate fastpath and
@@ -398,7 +461,7 @@ static bool mutex_optimistic_spin(struct mutex *lock, 
			break;



		/* Try to acquire the mutex if it is unlocked. */

		if (__mutex_trylock(lock)) {

		if (__mutex_trylock(lock, false)) {

			osq_unlock(&lock->osq);

			return true;

		}
@@ -523,6 +586,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
	struct task_struct *task = current;

	struct mutex_waiter waiter;

	unsigned long flags;

	bool first = false;

	int ret;



	if (use_ww_ctx) {
@@ -534,7 +598,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
	preempt_disable();

	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);



	if (__mutex_trylock(lock) || mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {

	if (__mutex_trylock(lock, false) ||

	    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {

		/* got the lock, yay! */

		lock_acquired(&lock->dep_map, ip);

		if (use_ww_ctx) {
@@ -551,7 +616,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
	/*

	 * After waiting to acquire the wait_lock, try again.

	 */

	if (__mutex_trylock(lock))

	if (__mutex_trylock(lock, false))

		goto skip_wait;



	debug_mutex_lock_common(lock, &waiter);
@@ -561,13 +626,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
	list_add_tail(&waiter.list, &lock->wait_list);

	waiter.task = task;



	if (list_first_entry(&lock->wait_list, struct mutex_waiter, list) == &waiter)

	if (__mutex_waiter_is_first(lock, &waiter))

		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);



	lock_contended(&lock->dep_map, ip);



	for (;;) {

		if (__mutex_trylock(lock))

		if (__mutex_trylock(lock, first))

			break;



		/*
@@ -586,17 +651,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
		}



		__set_task_state(task, state);



		/* didn't get the lock, go to sleep: */

		spin_unlock_mutex(&lock->wait_lock, flags);

		schedule_preempt_disabled();

		spin_lock_mutex(&lock->wait_lock, flags);



		if (!first && __mutex_waiter_is_first(lock, &waiter)) {

			first = true;

			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);

		}

	}

	__set_task_state(task, TASK_RUNNING);



	mutex_remove_waiter(lock, &waiter, task);

	if (likely(list_empty(&lock->wait_list)))

		__mutex_clear_flag(lock, MUTEX_FLAG_WAITERS);

		__mutex_clear_flag(lock, MUTEX_FLAGS);



	debug_mutex_free_waiter(&waiter);
@@ -724,33 +792,61 @@ EXPORT_SYMBOL_GPL(__ww_mutex_lock_interruptible); 
 */

static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)

{

	struct task_struct *next = NULL;

	unsigned long owner, flags;

	WAKE_Q(wake_q);



	mutex_release(&lock->dep_map, 1, ip);



	/*

	 * Release the lock before (potentially) taking the spinlock

	 * such that other contenders can get on with things ASAP.

	 * Release the lock before (potentially) taking the spinlock such that

	 * other contenders can get on with things ASAP.

	 *

	 * Except when HANDOFF, in that case we must not clear the owner field,

	 * but instead set it to the top waiter.

	 */

	owner = atomic_long_fetch_and_release(MUTEX_FLAGS, &lock->owner);

	if (!__owner_flags(owner))

	owner = atomic_long_read(&lock->owner);

	for (;;) {

		unsigned long old;



#ifdef CONFIG_DEBUG_MUTEXES

		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);

#endif



		if (owner & MUTEX_FLAG_HANDOFF)

			break;



		old = atomic_long_cmpxchg_release(&lock->owner, owner,

						  __owner_flags(owner));

		if (old == owner) {

			if (owner & MUTEX_FLAG_WAITERS)

				break;



			return;

		}



		owner = old;

	}



	spin_lock_mutex(&lock->wait_lock, flags);

	debug_mutex_unlock(lock);



	if (!list_empty(&lock->wait_list)) {

		/* get the first entry from the wait-list: */

		struct mutex_waiter *waiter =

				list_entry(lock->wait_list.next,

			list_first_entry(&lock->wait_list,

					 struct mutex_waiter, list);



		next = waiter->task;



		debug_mutex_wake_waiter(lock, waiter);

		wake_q_add(&wake_q, waiter->task);

		wake_q_add(&wake_q, next);

	}



	if (owner & MUTEX_FLAG_HANDOFF)

		__mutex_handoff(lock, next);



	spin_unlock_mutex(&lock->wait_lock, flags);



	wake_up_q(&wake_q);

}
@@ -853,7 +949,7 @@ __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock, 
 */

int __sched mutex_trylock(struct mutex *lock)

{

	bool locked = __mutex_trylock(lock);

	bool locked = __mutex_trylock(lock, false);



	if (locked)

		mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);