On Wed, Jan 19, 2011 at 10:52 AM, Jan Kara <jack@xxxxxxx> wrote: > On Wed 19-01-11 09:17:23, Nick Piggin wrote: >> On Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <jack@xxxxxxx> wrote: >> > On Tue 18-01-11 10:24:24, Nick Piggin wrote: >> >> On Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: >> >> > Nick Piggin <npiggin@xxxxxxxxx> 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() hammer. 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 requests. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
