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Merge branch 'locking/urgent' into locking/core, to pick up dependency

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Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2016-06-08 14:35:29 +02:00
commit ae0b5c2f03
2 changed files with 77 additions and 36 deletions
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53
include/asm-generic/qspinlock.h
View file

@ -21,38 +21,34 @@
#include <asm-generic/qspinlock_types.h>
/**
* queued_spin_unlock_wait - wait until the _current_ lock holder releases the lock
* @lock : Pointer to queued spinlock structure
*
* There is a very slight possibility of live-lock if the lockers keep coming
* and the waiter is just unfortunate enough to not see any unlock state.
*/
#ifndef queued_spin_unlock_wait
extern void queued_spin_unlock_wait(struct qspinlock *lock);
#endif
/**
* queued_spin_is_locked - is the spinlock locked?
* @lock: Pointer to queued spinlock structure
* Return: 1 if it is locked, 0 otherwise
*/
#ifndef queued_spin_is_locked
static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
{
/*
* queued_spin_lock_slowpath() can ACQUIRE the lock before
* issuing the unordered store that sets _Q_LOCKED_VAL.
* See queued_spin_unlock_wait().
*
* See both smp_cond_acquire() sites for more detail.
*
* This however means that in code like:
*
* spin_lock(A) spin_lock(B)
* spin_unlock_wait(B) spin_is_locked(A)
* do_something() do_something()
*
* Both CPUs can end up running do_something() because the store
* setting _Q_LOCKED_VAL will pass through the loads in
* spin_unlock_wait() and/or spin_is_locked().
*
* Avoid this by issuing a full memory barrier between the spin_lock()
* and the loads in spin_unlock_wait() and spin_is_locked().
*
* Note that regular mutual exclusion doesn't care about this
* delayed store.
* Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
* isn't immediately observable.
*/
smp_mb();
return atomic_read(&lock->val) & _Q_LOCKED_MASK;
return atomic_read(&lock->val);
}
#endif
/**
* queued_spin_value_unlocked - is the spinlock structure unlocked?
@ -122,21 +118,6 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
}
#endif
/**
* queued_spin_unlock_wait - wait until current lock holder releases the lock
* @lock : Pointer to queued spinlock structure
*
* There is a very slight possibility of live-lock if the lockers keep coming
* and the waiter is just unfortunate enough to not see any unlock state.
*/
static inline void queued_spin_unlock_wait(struct qspinlock *lock)
{
/* See queued_spin_is_locked() */
smp_mb();
while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
cpu_relax();
}
#ifndef virt_spin_lock
static __always_inline bool virt_spin_lock(struct qspinlock *lock)
{

60
kernel/locking/qspinlock.c
View file

@ -267,6 +267,66 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
#define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
#endif
/*
* queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before
* issuing an _unordered_ store to set _Q_LOCKED_VAL.
*
* This means that the store can be delayed, but no later than the
* store-release from the unlock. This means that simply observing
* _Q_LOCKED_VAL is not sufficient to determine if the lock is acquired.
*
* There are two paths that can issue the unordered store:
*
* (1) clear_pending_set_locked(): *,1,0 -> *,0,1
*
* (2) set_locked(): t,0,0 -> t,0,1 ; t != 0
* atomic_cmpxchg_relaxed(): t,0,0 -> 0,0,1
*
* However, in both cases we have other !0 state we've set before to queue
* ourseves:
*
* For (1) we have the atomic_cmpxchg_acquire() that set _Q_PENDING_VAL, our
* load is constrained by that ACQUIRE to not pass before that, and thus must
* observe the store.
*
* For (2) we have a more intersting scenario. We enqueue ourselves using
* xchg_tail(), which ends up being a RELEASE. This in itself is not
* sufficient, however that is followed by an smp_cond_acquire() on the same
* word, giving a RELEASE->ACQUIRE ordering. This again constrains our load and
* guarantees we must observe that store.
*
* Therefore both cases have other !0 state that is observable before the
* unordered locked byte store comes through. This means we can use that to
* wait for the lock store, and then wait for an unlock.
*/
#ifndef queued_spin_unlock_wait
void queued_spin_unlock_wait(struct qspinlock *lock)
{
u32 val;
for (;;) {
val = atomic_read(&lock->val);
if (!val) /* not locked, we're done */
goto done;
if (val & _Q_LOCKED_MASK) /* locked, go wait for unlock */
break;
/* not locked, but pending, wait until we observe the lock */
cpu_relax();
}
/* any unlock is good */
while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
cpu_relax();
done:
smp_rmb(); /* CTRL + RMB -> ACQUIRE */
}
EXPORT_SYMBOL(queued_spin_unlock_wait);
#endif
#endif /* _GEN_PV_LOCK_SLOWPATH */
/**