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Date:   Thu, 31 Jan 2019 01:27:25 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
cc:     Sebastian Sewior <bigeasy@linutronix.de>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-s390@vger.kernel.org,
        Stefan Liebler <stli@linux.ibm.com>
Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggerede
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On Thu, 31 Jan 2019, 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 ...

And of course, I'm using the latest greatest glibc for testing that, so I'm
not at all surprised that it just does not reproduce on my tests.

I just hacked the ordering and restarted the test. If the theory holds,
then this should die sooner than later.

Thanks,

	tglx