linux-kernel - Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit

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Message-ID: <20160901221743.GJ10168@twins.programming.kicks-ass.net>
Date:   Fri, 2 Sep 2016 00:17:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...IV.linux.org.uk>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Neil Brown <neilb@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in
 __wait_on_bit_lock()

On Thu, Sep 01, 2016 at 09:01:41PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 26, 2016 at 02:45:52PM +0200, Oleg Nesterov wrote:
> 
> > We do not need anything tricky to avoid the race,
> 
> The race being:
> 
> CPU0			CPU1			CPU2
> 			
> 			__wait_on_bit_lock()
> 			  bit_wait_io()
> 			    io_schedule()
> 
> clear_bit_unlock()
> __wake_up_common(.nr_exclusive=1)
>   list_for_each_entry()
>     if (curr->func() && --nr_exclusive)
>       break
> 
> 						signal()
> 
> 			    if (signal_pending_state()) == TRUE
> 			      return -EINTR
> 
> And no progress because CPU1 exits without acquiring the lock and CPU0
> thinks its done because it woke someone.

FWIW, the way the mutex code avoids this issue is by doing the
signal_pending test while holding the q->lock, that way its exclusive
with wakeup.