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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: Jason Low <jason.low2@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman 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: Ingo Molnar <mingo@kernel.org>
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Peter Zijlstra Ingo Molnar
Peter Zijlstra authored and Ingo Molnar committed Aug 23, 2016
1 parent a3ea3d9 commit 9d659ae14b545c4296e812c70493bfdc999b5c1c
Showing with 119 additions and 23 deletions.
  1. +119 −23 kernel/locking/mutex.c
@@ -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;
}

/*
* 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 | __owner_flags(owner));
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))
return;
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,
struct mutex_waiter, list);
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_);

2 comments on commit 9d659ae

@schadfield

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replied Mar 16, 2018

Are you aware this commit has been implicated in an issue with a number of laptops using the i8042 for keyboard and trackpad? https://bugzilla.kernel.org/show_bug.cgi?id=195471 . That bug has not been fixed yet.

@smp79

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replied Oct 16, 2018

It also introduced issues with some DVB cards - https://bugzilla.kernel.org/show_bug.cgi?id=199323

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