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    SubjectRe: [patch] fs: aio fix rcu lookup
    On Wed, Jan 19, 2011 at 10:52 AM, Jan Kara <> wrote:
    > On Wed 19-01-11 09:17:23, Nick Piggin wrote:
    >> On Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <> wrote:
    >> > On Tue 18-01-11 10:24:24, Nick Piggin wrote:
    >> >> On Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <> wrote:
    >> >> > Nick Piggin <> 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...
    >> With my fix we won't oops, I was a bit concerned about ->dead,
    >> yes but I don't know what semantics it is attempted to have there.
    >  But wouldn't it do something bad if the memory gets reallocated for
    > something else and set to non-zero? E.g. memory corruption?

    I don't see how it would with my patch.

    >> synchronize_rcu() in io_destroy()  does not prevent it from returning
    >> as soon as lookup_ioctx drops the rcu_read_lock().
    >  Yes, exactly. So references obtained before synchronize_rcu() would be
    > completely fine and valid references and there would be no references after
    > synchronize_rcu() because they'd see 'dead' set. But looking at the code
    > again it still would not be enough because we could still race with
    > io_submit_one() adding new IO to the ioctx which will be freed just after
    > put_ioctx() in do_io_submit().
    > The patch below implements what I have in mind - it should be probably
    > split into two but I'd like to hear comments on that before doing these
    > cosmetic touches ;)

    Well this seems to solve it too, but it is 2 problems here. It is changing
    the semantics of io_destroy which requires the big synchronize_rcu()

    But I don't believe that's necessarily desirable, or required. In fact it is
    explicitly not reuired because it only says that it _may_ cancel outstanding
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      Last update: 2011-01-19 01:23    [W:0.026 / U:7.352 seconds]
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