linux-kernel - Re: [RFC v7] wait: prevent exclusive waiter starvation

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Date:	Thu, 29 Jan 2009 01:11:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matthew Wilcox <matthew@....cx>,
	Chuck Lever <cel@...i.umich.edu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>, stable@...nel.org
Subject: Re: [RFC v7] wait: prevent exclusive waiter starvation

On Thu, 29 Jan 2009 09:31:08 +0100 Oleg Nesterov <oleg@...hat.com> wrote:

> On 01/28, Andrew Morton wrote:
> >
> > On Thu, 29 Jan 2009 05:42:27 +0100 Oleg Nesterov <oleg@...hat.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,
> 
> > And that "[RFC v7] wait: prevent exclusive waiter starvation" fixes it
> > as well, and in a preferable manner, but not a manner which we consider
> > suitable for -stable?  (why?)
> 
> I meant that lock_page_killable-avoid-lost-wakeups.patch is much simpler,
> and thus it looks more "safe" for -stable.
> 
> But it is not optimal, and Johannes's patch is also more generic, it fixes
> wait_event_interruptible_exclusive() as well.
> 
> > And hence that lock_page_killable-avoid-lost-wakeups.patch is the
> > appropriate fix for -stable?
> >
> > If so, that's a bit unusual, and the -stable maintainers may choose to
> > take the patch which we're putting into 2.6.29.
> 
> Well, I don't know ;)
> 
> But Johannes's looks good to me, if you already dropped the old patch,
> then I think this one can go into -stable after some testing. Hopefully
> maintainers can review it.
> 

OK, thanks.  That's why we pay the stable guys the big bucks.

I tagged the patch with

Cc: <stable@...nel.org>         ["after some testing"]

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