Re: [RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg()

[Xu, Yanfei]

On 7/5/21 10:00 PM, Peter Zijlstra wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> 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@xxxxxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxxxxx>
> > 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.

Got it. Thanks!

Yanfei
>