linux-kernel - Re: [PATCH] locking/rtmutex: remove unused cmpxchg

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Date:   Fri, 31 Jan 2020 09:39:22 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

Cc'ing tglx.

On Tue, 21 Jan 2020, Alex Shi wrote:

>No one use this macro after it was introduced. Better to remove it?

You also need to remove it for the CONFIG_DEBUG_RT_MUTEXES=y case.

>
>Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
>Cc: Peter Zijlstra <peterz@...radead.org>
>Cc: Davidlohr Bueso <dave@...olabs.net>
>Cc: Ingo Molnar <mingo@...hat.com>
>Cc: Will Deacon <will@...nel.org>
>Cc: linux-kernel@...r.kernel.org
>---
> kernel/locking/rtmutex.c | 1 -
> 1 file changed, 1 deletion(-)
>
>diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>index 851bbb10819d..a849964a348d 100644
>--- a/kernel/locking/rtmutex.c
>+++ b/kernel/locking/rtmutex.c
>@@ -141,7 +141,6 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>  * set up.
>  */
> #ifndef CONFIG_DEBUG_RT_MUTEXES
>-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
> # define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> # define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)

Hmm unrelated, but do we want CCAS for rtmutex fastpath? Ie:

     (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)

That would optimize for the contended case and avoid the cmpxchg - it would
also help if we ever do the top-waiter spin thing.

Thanks,
Davidlohr