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Message-ID: <feb68b86-899e-42a2-a680-4a73366e540d@redhat.com>
Date: Thu, 21 Mar 2024 19:52:21 -0400
From: Waiman Long <longman@...hat.com>
To: Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in
xchg_tail()
On 3/21/24 15:52, Uros Bizjak wrote:
> Use atomic_try_cmpxchg_relaxed(*ptr, &old, new) instead of
> atomic_cmpxchg_relaxed (*ptr, old, new) == old in xchg_tail().
> x86 CMPXCHG instruction returns success in ZF flag,
> so this change saves a compare after cmpxchg.
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> ---
> kernel/locking/qspinlock.c | 13 +++++--------
> 1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index ebe6b8ec7cb3..1df5fef8a656 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -220,21 +220,18 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
> */
> static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> {
> - u32 old, new, val = atomic_read(&lock->val);
> + u32 old, new;
>
> - for (;;) {
> - new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> + old = atomic_read(&lock->val);
> + do {
> + new = (old & _Q_LOCKED_PENDING_MASK) | tail;
> /*
> * We can use relaxed semantics since the caller ensures that
> * the MCS node is properly initialized before updating the
> * tail.
> */
> - old = atomic_cmpxchg_relaxed(&lock->val, val, new);
> - if (old == val)
> - break;
> + } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
>
> - val = old;
> - }
> return old;
> }
> #endif /* _Q_PENDING_BITS == 8 */
LGTM, note that this xchg_tail() variant is not used in all the distros
that I am aware of as it requires NR_CPUS >= 16k.
Reviewed-by: Waiman Long <longman@...hat.com>
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