linux-kernel - Re: [BUG] futex_unlock_pi() hurts my brain and may cause application deadlock

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Message-ID: <a36005b50705310753x2a09172boa6ceb63f9ca4d4b8@mail.gmail.com>
Date:	Thu, 31 May 2007 07:53:49 -0700
From:	"Ulrich Drepper" <drepper@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"john stultz" <johnstul@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Sripathi Kodi" <sripathik@...ibm.com>
Subject: Re: [BUG] futex_unlock_pi() hurts my brain and may cause application deadlock

On 5/30/07, Steven Rostedt <rostedt@...dmis.org> wrote:
> >       if (!(uval & FUTEX_OWNER_DIED)) {
> >               pagefault_disable();
> >               uval = futex_atomic_cmpxchg_inatomic(uaddr, current->pid, 0);
> >               pagefault_enable();
> >       }
> [...]
> This code is in futex_unlock_pi.  Can the owner of the mutex really die?
> Isn't the owner the one doing the unlock?

This is part of the implementation of robust PI futexes.  The
semantics is that if the owner of a robust futex dies the futex is
marked with FUTEX_OWNER_DIED.  The next locking thread then has to
clear that bit before calling pthread_mutex_unlock.  If the bit is
still set while unlocking the mutex is permanently marked unusable.  I
cannot say right now whether this is the semantics implemented above.
I'll have to check the glibc test suite whether we check for that
semantic.
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