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SubjectRe: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb()
On Thu, Apr 05, 2018 at 05:59:07PM +0100, Will Deacon wrote:
> 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.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> kernel/locking/qspinlock.c | 32 +++++++++++++++-----------------
> 1 file changed, 15 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 3ad8786a47e2..42c61f7b37c5 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -141,10 +141,10 @@ 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;
> }
>
> @@ -178,10 +178,11 @@ 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;
>
> @@ -340,12 +341,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> goto release;
>
> /*
> + * Ensure that the initialisation of @node is complete before we
> + * publish the updated tail and potentially link @node into the

I think it might be better if we mention exactly where we "publish the
updated tail" and "link @node", how about:

* publish the update tail via xchg_tail() and potentially link
* @node into the waitqueue via WRITE_ONCE(->next,..) below.

and also add comments below like:

> + * waitqueue.
> + */
> + smp_wmb();
> +
> + /*
> * 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.

* publish the updated tail

> */
> old = xchg_tail(lock, tail);
> next = NULL;
> @@ -356,15 +362,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> */
> 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);

/* Eventually link @node to the wait queue */

Thoughts?

Regards,
Boqun

> + WRITE_ONCE(prev->next, node);
>
> pv_wait_node(node, prev);
> arch_mcs_spin_lock_contended(&node->locked);
> --
> 2.1.4
>
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