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

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Message-ID: <20110118190114.GA5070@quack.suse.cz>
Date:	Tue, 18 Jan 2011 20:01:14 +0100
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <npiggin@...il.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	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

  Hi,

On Tue 18-01-11 10:24:24, Nick Piggin wrote:
> On Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> > Nick Piggin <npiggin@...il.com> writes:
> >> Do you agree with the theoretical problem? I didn't try to
> >> write a racer to break it yet. Inserting a delay before the
> >> get_ioctx might do the trick.
> >
> > I'm not convinced, no.  The last reference to the kioctx is always the
> > process, released in the exit_aio path, or via sys_io_destroy.  In both
> > cases, we cancel all aios, then wait for them all to complete before
> > dropping the final reference to the context.
> 
> That wouldn't appear to prevent a concurrent thread from doing an
> io operation that requires ioctx lookup, and taking the last reference
> after the io_cancel thread drops the ref.
> 
> > So, while I agree that what you wrote is better, I remain unconvinced of
> > it solving a real-world problem.  Feel free to push it in as a cleanup,
> > though.
> 
> Well I think it has to be technically correct first. If there is indeed a
> guaranteed ref somehow, it just needs a comment.
  Hmm, the code in io_destroy() indeed looks fishy. We delete the ioctx
from the hash table and set ioctx->dead which is supposed to stop
lookup_ioctx() from finding it (see the !ctx->dead check in
lookup_ioctx()). There's even a comment in io_destroy() saying:
        /*
         * Wake up any waiters.  The setting of ctx->dead must be seen
         * by other CPUs at this point.  Right now, we rely on the
         * locking done by the above calls to ensure this consistency.
         */
But since lookup_ioctx() is called without any lock or barrier nothing
really seems to prevent the list traversal and ioctx->dead test to happen
before io_destroy() and get_ioctx() after io_destroy().

But wouldn't the right fix be to call synchronize_rcu() in io_destroy()?
Because with your fix we could still return 'dead' ioctx and I don't think
we are supposed to do that...

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