linux-kernel - Re: [RFC][PATCH] muptiple bugs in PI futexes

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Date:	Wed, 6 Jun 2007 01:00:28 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: [RFC][PATCH] muptiple bugs in PI futexes

Hello!

> We actually need to do something about this, as we might loop for ever
> there. The robust cleanup code can fail (e.g. due to list corruption)
> and we would see exit_state != 0 and the OWNER_DIED bit would never be
> set, so we are stuck in a busy loop.

Yes...

It is possible to take read_lock(&tasklist_lock) before:

        inc_preempt_count();
        curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
        dec_preempt_count();

and drop it after lookup_pi_state().

In this case exiting task will set FUTEX_OWNER_DIED, but will
spin in exit_notify(), we will find valid pi_state and go slow path
taking rtmutex.

Alexey
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