linux-kernel - Re: commit cfafcd117 "futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()" causes glibc nptl/tst-robustpi8 failure

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Date:   Thu, 18 May 2017 08:46:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Markus Trippelsdorf <markus@...ppelsdorf.de>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Torvald Riegel <triegel@...hat.com>
Subject: Re: commit cfafcd117 "futex: Rework futex_lock_pi() to use
 rt_mutex_*_proxy_lock()" causes glibc nptl/tst-robustpi8 failure

On Wed, May 17, 2017 at 07:50:31PM +0200, Florian Weimer wrote:
> On 05/17/2017 07:36 PM, Markus Trippelsdorf wrote:
> > Since:
> > commit cfafcd117da0216520568c195cb2f6cd1980c4bb
> > Author: Peter Zijlstra <peterz@...radead.org>
> > Date:   Wed Mar 22 11:35:58 2017 +0100
> > 
> >     futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()
> > 
> > glibc's nptl/tst-robustpi8 testcase fails:
> > 
> > glibc-build % ./nptl/tst-robustpi8
> > tst-robustpi8: ../nptl/pthread_mutex_lock.c:424: __pthread_mutex_lock_full: Assertion `INTERNAL_SYSCALL_ERRNO (e, __err) != ESRCH || !robust' failed.
> > 
> > pthread_mutex_lock.c:
> > 415             if (INTERNAL_SYSCALL_ERROR_P (e, __err)
> > 416                 && (INTERNAL_SYSCALL_ERRNO (e, __err) == ESRCH
> > 417                     || INTERNAL_SYSCALL_ERRNO (e, __err) == EDEADLK))
> > 418               {
> > 419                 assert (INTERNAL_SYSCALL_ERRNO (e, __err) != EDEADLK
> > 420                         || (kind != PTHREAD_MUTEX_ERRORCHECK_NP
> > 421                             && kind != PTHREAD_MUTEX_RECURSIVE_NP));
> > 422                 /* ESRCH can happen only for non-robust PI mutexes where
> > 423                    the owner of the lock died.  */
> > 424                 assert (INTERNAL_SYSCALL_ERRNO (e, __err) != ESRCH || !robust);
> > 
> > During bisection the commit above hangs the machine when I run the
> > testcase.
> > 
> > See: https://sourceware.org/bugzilla/show_bug.cgi?id=21487
> 
> Markus, could you confirm that it is chocking on the EAGAIN failure?  Or
> is it something else?
> 
> What is userspace supposed to do with the error code?

IIRC that -EAGAIN should not get to userspace. The kernel should retry
the lock operation. I'll go stare at it.