linux-kernel - Re: WARN_ON_ONCE(!new_owner) within wake_futex

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Date:   Wed, 30 Jan 2019 17:44:42 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Sewior <bigeasy@...utronix.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-s390@...r.kernel.org,
        Stefan Liebler <stli@...ux.ibm.com>
Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggerede

On Thu, Jan 31, 2019 at 12:55:18AM +0100, Thomas Gleixner wrote:
> On Wed, 30 Jan 2019, Paul E. McKenney wrote:
> > On Thu, Jan 31, 2019 at 12:13:51AM +0100, Thomas Gleixner wrote:
> > > I might be wrong as usual, but this would definitely explain the fail very
> > > well.
> > 
> > On recent versions of GCC, the fix would be to put this between the two
> > stores that need ordering:
> > 
> > 	__atomic_thread_fence(__ATOMIC_RELEASE);
> > 
> > I must defer to Heiko on whether s390 GCC might tear the stores.  My
> > guess is "probably not".  ;-)
> 
> So I just checked the latest glibc code. It has:
> 
>             /* We must not enqueue the mutex before we have acquired it.
>                Also see comments at ENQUEUE_MUTEX.  */
>             __asm ("" ::: "memory");
>             ENQUEUE_MUTEX_PI (mutex);
>             /* We need to clear op_pending after we enqueue the mutex.  */
>             __asm ("" ::: "memory");
>             THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);
> 
> 8f9450a0b7a9 ("Add compiler barriers around modifications of the robust mutex list.")
> 
> in the glibc repository, There since Dec 24 2016 ...
> 
> So the question is whether this is sufficient. That ordering only only
> matters vs. the thread itself and not for others.

Ah, in that case you can use __atomic_signal_fence(__ATOMIC_RELEASE)
instead of the __atomic_thread_fence(__ATOMIC_RELEASE).

The __atomic_thread_fence(__ATOMIC_RELEASE) provides ordering between
threads, but __atomic_signal_fence(__ATOMIC_RELEASE) only does so
within a thread.

							Thanx, Paul