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Date: Fri, 19 May 2017 18:07:13 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Darren Hart <dvhart@infradead.org>
Subject: Re: commit cfafcd117 "futex: Rework futex_lock_pi() to use
 rt_mutex_*_proxy_lock()" causes glibc nptl/tst-robustpi8 failure
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On Fri, May 19, 2017 at 05:48:50PM +0200, Peter Zijlstra wrote:

> Markus reported that the glibc/nptl/tst-robustpi8 test was failing after
> commit:
> 
>   cfafcd117da0 ("futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()")
> 
> Much tracing later I managed to catch the culprit:
> 
>  ld-linux-x86-64-2161  [019] ....   410.760971: SyS_futex: 00007ffbeb76b028: 80000875  op=FUTEX_LOCK_PI
>  ld-linux-x86-64-2161  [019] ...1   410.760972: lock_pi_update_atomic: 00007ffbeb76b028: curval=80000875 uval=80000875 newval=80000875 ret=0
>  ld-linux-x86-64-2165  [011] ....   410.760978: SyS_futex: 00007ffbeb76b028: 80000875  op=FUTEX_UNLOCK_PI
>  ld-linux-x86-64-2165  [011] d..1   410.760979: do_futex: 00007ffbeb76b028: curval=80000875 uval=80000875 newval=80000871 ret=0
>  ld-linux-x86-64-2165  [011] ....   410.760980: SyS_futex: 00007ffbeb76b028: 80000871 ret=0000
>  ld-linux-x86-64-2161  [019] ....   410.760980: SyS_futex: 00007ffbeb76b028: 80000871 ret=ETIMEDOUT

The above trace continues like:

 ld-linux-x86-64-2164  [006] ....   410.762336: SyS_futex: 00007ffbeb76b028: 80000871  op=FUTEX_LOCK_PI
 ld-linux-x86-64-2164  [006] ...1   410.762337: lock_pi_update_atomic: 00007ffbeb76b028: curval=80000871 uval=80000871 newval=80000871 ret=0
 ld-linux-x86-64-2164  [006] ....   410.762347: SyS_futex: 00007ffbeb76b028: 80000871 ret=ETIMEDOUT
 ld-linux-x86-64-2161  [019] ....   410.762521: SyS_futex: 00007ffbeb76b028: 80000871  op=FUTEX_LOCK_PI
 ld-linux-x86-64-2161  [019] ....   410.762522: SyS_futex: 00007ffbeb76b028: 80000871 ret=EDEADLK

And every subsequent attempt by 2161 will (obviously) return EDEADLK.

Now since the test explicitly tracks the lock state[] and
pthread_mutex_*lock() return values this _should_ have triggered one of
the printf()'s, but I never saw any of those.