Subject: Re: [RFC v7] wait: prevent exclusive waiter starvation
From: Chris Mason <chris.mason@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Matthew Wilcox <matthew@wil.cx>, Chuck Lever <cel@citi.umich.edu>,
       Nick Piggin <nickpiggin@yahoo.com.au>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Ingo Molnar <mingo@elte.hu>, stable@kernel.org
In-Reply-To: <20090129011143.884e5573.akpm@linux-foundation.org>
References: <20090123095904.GA22890@cmpxchg.org>
	 <20090123113541.GB12684@redhat.com> <20090123133050.GA19226@redhat.com>
	 <20090126215957.GA3889@cmpxchg.org> <20090127032359.GA17359@redhat.com>
	 <20090127193434.GA19673@cmpxchg.org> <20090127200544.GA28843@redhat.com>
	 <20090128091453.GA22036@cmpxchg.org> <20090129044227.GA5231@redhat.com>
	 <20090128233734.81d8004a.akpm@linux-foundation.org>
	 <20090129083108.GA27495@redhat.com>
	 <20090129011143.884e5573.akpm@linux-foundation.org>
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Date: Thu, 29 Jan 2009 09:34:35 -0500
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On Thu, 2009-01-29 at 01:11 -0800, Andrew Morton wrote:
> On Thu, 29 Jan 2009 09:31:08 +0100 Oleg Nesterov <oleg@redhat.com> wrote:
> 
> > On 01/28, Andrew Morton wrote:
> > >
> > > On Thu, 29 Jan 2009 05:42:27 +0100 Oleg Nesterov <oleg@redhat.com> wrote:
> > >
> > > > On 01/28, Johannes Weiner wrote:
> > > > >
> > > > > Add abort_exclusive_wait() which removes the process' wait descriptor
> > > > > from the waitqueue, iff still queued, or wakes up the next waiter
> > > > > otherwise.  It does so under the waitqueue lock.  Racing with a wake
> > > > > up means the aborting process is either already woken (removed from
> > > > > the queue) and will wake up the next waiter, or it will remove itself
> > > > > from the queue and the concurrent wake up will apply to the next
> > > > > waiter after it.
> > > > >
> > > > > Use abort_exclusive_wait() in __wait_event_interruptible_exclusive()
> > > > > and __wait_on_bit_lock() when they were interrupted by other means
> > > > > than a wake up through the queue.
> > > >
> > > > Imho, this all is right, and this patch should replace
> > > > lock_page_killable-avoid-lost-wakeups.patch (except for stable tree).
> > >
> > > I dropped lock_page_killable-avoid-lost-wakeups.patch a while ago.
> > >
> > > So I think we're saying that
> > > lock_page_killable-avoid-lost-wakeups.patch actually did fix the bug?
> > 
> > I think yes,
> > 

Our test case that was able to reliably trigger the bug was fixed by
lock_page_killable-avoid-lost-wakeups.patch.

I'll ask them to test v7 as well.  The run takes about a day, so
confirmation will take a bit.

-chris


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