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

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Date:	Thu, 29 Jan 2009 09:31:08 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 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.

Oleg.

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