linux-kernel - Re: [RFC][PATCH 1/4] locking/mutex: Use try

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Date:   Mon, 5 Jul 2021 16:00:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Yanfei" <yanfei.xu@...driver.com>
Cc:     mingo@...hat.com, longman@...hat.com, boqun.feng@...il.com,
        will@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg()

On Mon, Jul 05, 2021 at 07:59:12PM +0800, Xu, Yanfei wrote:
> 
> 
> On 6/30/21 11:35 PM, Peter Zijlstra wrote:
> > For simpler and better code.
> > 
> > Signed-off-by: Peter Zijlstra (Intel)<peterz@...radead.org>
> > ---
> >   kernel/locking/mutex.c |   27 ++++++---------------------
> >   1 file changed, 6 insertions(+), 21 deletions(-)
> 
> Hi Peter,
> 
> I read the mutex codes today, and find there seems something wrong for the
> patch. Should we consider the race condition as blow?
> 
> From 4035f50c96e17cbe3febab768b64da5c000e5b76 Mon Sep 17 00:00:00 2001
> From: Yanfei Xu <yanfei.xu@...driver.com>
> Date: Mon, 5 Jul 2021 17:56:58 +0800
> Subject: [PATCH] locking/mutex: fix the endless loop when racing against
>  mutex.owner
> 
> if a race condition happened on mutex.owner after we fetch its value,
> atomic_long_try_cmpxchg_acquire/release invoked on &mutex.owner will
> return false. Then we need to reassign the temporary variable which
> saves mutex.owner value if in loop, or it will lead an endless loop.

No, when try_cmpxchg() fails it will update oldp. This is the reason old
is now a pointer too.