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

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Date:   Wed, 12 Feb 2020 07:07:56 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

On Tue, 04 Feb 2020, Alex Shi wrote:

>Thanks Thomas and David!
>Is this following patch ok?

So if anything, this really wants to be two patches.

>---
>>From 4cf9e38a73c67c6894f3addb2ddca26bb51b1a28 Mon Sep 17 00:00:00 2001
>From: Alex Shi <alex.shi@...ux.alibaba.com>
>Date: Tue, 21 Jan 2020 15:03:33 +0800
>Subject: [PATCH v2] locking/rtmutex: optimize rt_mutex_cmpxchg_xxx series func
>
>rt_mutex_cmpxchg_relexed isn't interested by anyone, so remove it.
>And Davidlohr Bueso suggests check l->owner before cmpxchg to reduce
>lock contention.
>
>Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
>Cc: Thomas Gleixner <tglx@...utronix.de>
>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 | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
>diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>index 851bbb10819d..eb26f4e57ce4 100644
>--- a/kernel/locking/rtmutex.c
>+++ b/kernel/locking/rtmutex.c
>@@ -141,9 +141,10 @@ 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)
>+# define rt_mutex_cmpxchg_acquire(l,c,n)	\
>+		(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
>+# define rt_mutex_cmpxchg_release(l,c,n)	\
>+		(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)

Thomas, should I resend the top-waiter spin series? Otherwise yeah,
I see little point to the CCAS fastpath thing.

Thanks,
Davidlohr