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

[Peter Zijlstra]

On Thu, May 18, 2017 at 08:46:17AM +0200, Peter Zijlstra wrote:
> 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@xxxxxxxxxxxxx>
> > > 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.

So commit:

  bebe5b514345 ("futex: Futex_unlock_pi() determinism")

put a WARN_ON_ONCE() on that -EAGAIN condition, and since that doesn't
appear to be triggering, I suspect something else is buggered.