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locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb()

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The qspinlock slowpath must ensure that the MCS node is fully initialised
before it can be reached by another other CPU. This is currently enforced
by using a RELEASE operation when updating the tail and also when linking
the node into the waitqueue, since the control dependency off xchg_tail
is insufficient to enforce sufficient ordering, see:

  95bcade33a8a ("locking/qspinlock: Ensure node is initialised before updating prev->next")

Back-to-back RELEASE operations may be expensive on some architectures,
particularly those that implement them using fences under the hood. We
can replace the two RELEASE operations with a single smp_wmb() fence and
use RELAXED operations for the subsequent publishing of the node.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-12-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>
This commit is contained in:
Will Deacon 2018-04-26 11:34:25 +01:00 committed by Adithya R
parent bbc29076d2
commit 94f9902b78
1 changed files with 17 additions and 16 deletions
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33
kernel/locking/qspinlock.c
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@ -164,10 +164,10 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
{
/*
* Use release semantics to make sure that the MCS node is properly
* initialized before changing the tail code.
* We can use relaxed semantics since the caller ensures that the
* MCS node is properly initialized before updating the tail.
*/
return (u32)xchg_release(&lock->tail,
return (u32)xchg_relaxed(&lock->tail,
tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
}
@ -212,10 +212,11 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
for (;;) {
new = (val & _Q_LOCKED_PENDING_MASK) | tail;
/*
* Use release semantics to make sure that the MCS node is
* properly initialized before changing the tail code.
* We can use relaxed semantics since the caller ensures that
* the MCS node is properly initialized before updating the
* tail.
*/
old = atomic_cmpxchg_release(&lock->val, val, new);
old = atomic_cmpxchg_relaxed(&lock->val, val, new);
if (old == val)
break;
@ -388,12 +389,18 @@ queue:
goto release;
/*
* Ensure that the initialisation of @node is complete before we
* publish the updated tail via xchg_tail() and potentially link
* @node into the waitqueue via WRITE_ONCE(prev->next, node) below.
*/
smp_wmb();
/*
* Publish the updated tail.
* We have already touched the queueing cacheline; don't bother with
* pending stuff.
*
* p,*,* -> n,*,*
*
* RELEASE, such that the stores to @node must be complete.
*/
old = xchg_tail(lock, tail);
next = NULL;
@ -405,14 +412,8 @@ queue:
if (old & _Q_TAIL_MASK) {
prev = decode_tail(old);
/*
* We must ensure that the stores to @node are observed before
* the write to prev->next. The address dependency from
* xchg_tail is not sufficient to ensure this because the read
* component of xchg_tail is unordered with respect to the
* initialisation of @node.
*/
smp_store_release(&prev->next, node);
/* Link @node into the waitqueue. */
WRITE_ONCE(prev->next, node);
pv_wait_node(node, prev);
arch_mcs_spin_lock_contended(&node->locked);