linux-kernel - Re: [patch] fs: aio fix rcu lookup

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Message-ID: <20110120201602.GA19797@quack.suse.cz>
Date:	Thu, 20 Jan 2011 21:16:02 +0100
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <npiggin@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, Jeff Moyer <jmoyer@...hat.com>,
	Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fs: aio fix rcu lookup

On Fri 21-01-11 05:31:53, Nick Piggin wrote:
> On Thu, Jan 20, 2011 at 3:03 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Thu, Jan 20, 2011 at 08:20:00AM +1100, Nick Piggin wrote:
> >> On Thu, Jan 20, 2011 at 8:03 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> >> >> I don't know exactly how all programs use io_destroy -- of the small
> >> >> number that do, probably an even smaller number would care here. But I
> >> >> don't think it simplifies things enough to use synchronize_rcu for it.
> >> >
> >> > Above it sounded like you didn't think AIO should be using RCU at all.
> >>
> >> synchronize_rcu of course, not RCU (typo).
> >
> > I think that Nick is suggesting that call_rcu() be used instead.
> > Perhaps also very sparing use of synchronize_rcu_expedited(), which
> > is faster than synchronize_rcu(), but which which uses more CPU time.
> 
> call_rcu() is the obvious alternative, yes.
> 
> Basically, once we give in to synchronize_rcu() we're basically giving
> up. That's certainly a very good tradeoff for something like filesystem
> unregistration or module unload, it buys big simplifications in real
> fastpaths. But I just don't think it should be taken lightly.
So in the end, I've realized I don't need synchronize_rcu() at all and
in fact everything is OK even without call_rcu() if I base my fix on top
of your patch.

Attached is your patch with added comment I proposed and also a patch
fixing the second race. Better?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-fs-Fix-aio-rcu-ioctx-lookup.patch" of type "text/x-patch" (2358 bytes)

View attachment "0002-fs-Fix-race-between-io_destroy-and-io_submit-in-AIO.patch" of type "text/x-patch" (1828 bytes)