Nick Piggin <npiggin@xxxxxxxxx> writes: > On Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <jack@xxxxxxx> wrote: >> ÂHi, >> >> 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. ÂThe last reference to the kioctx is always the >>> > process, released in the exit_aio path, or via sys_io_destroy. ÂIn 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. ÂFeel 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. >> ÂHmm, 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: >> Â Â Â Â/* >> Â Â Â Â * Wake up any waiters. ÂThe setting of ctx->dead must be seen >> Â Â Â Â * by other CPUs at this point. ÂRight now, we rely on the >> Â Â Â Â * locking done by the above calls to ensure this consistency. >> Â Â Â Â */ >> 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. > > synchronize_rcu() in io_destroy() does not prevent it from returning > as soon as lookup_ioctx drops the rcu_read_lock(). > > The dead=1 in io_destroy indeed doesn't guarantee a whole lot. > Anyone know? See the comment above io_destroy for starters. Note that rcu was bolted on later, and I believe that ->dead has nothing to do with the rcu-ification. Cheers, Jeff -- 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
