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locking/barriers: Replace smp_cond_acquire() with smp_cond_load_acquire()

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This new form allows using hardware assisted waiting.

Some hardware (ARM64 and x86) allow monitoring an address for changes,
so by providing a pointer we can use this to replace the cpu_relax()
with hardware optimized methods in the future.

Requested-by: Will Deacon <will.deacon@arm.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-04-04 10:57:12 +02:00 • committed by Ingo Molnar
commit 1f03e8d291
5 changed files with 31 additions and 18 deletions
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12
kernel/locking/qspinlock.c
View file

@ -475,7 +475,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* sequentiality; this is because not all clear_pending_set_locked()
* implementations imply full barriers.
*/
smp_cond_acquire(!(atomic_read(&lock->val) & _Q_LOCKED_MASK));
smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
/*
* take ownership and clear the pending bit.
@ -562,7 +562,7 @@ queue:
*
* The PV pv_wait_head_or_lock function, if active, will acquire
* the lock and return a non-zero value. So we have to skip the
* smp_cond_acquire() call. As the next PV queue head hasn't been
* smp_cond_load_acquire() call. As the next PV queue head hasn't been
* designated yet, there is no way for the locked value to become
* _Q_SLOW_VAL. So both the set_locked() and the
* atomic_cmpxchg_relaxed() calls will be safe.
@ -573,7 +573,7 @@ queue:
if ((val = pv_wait_head_or_lock(lock, node)))
goto locked;
smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));
locked:
/*
@ -593,9 +593,9 @@ locked:
break;
}
/*
* The smp_cond_acquire() call above has provided the necessary
* acquire semantics required for locking. At most two
* iterations of this loop may be ran.
* The smp_cond_load_acquire() call above has provided the
* necessary acquire semantics required for locking. At most
* two iterations of this loop may be ran.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)