On Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <jack@xxxxxxx> wrote: > Hi, > > 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. 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? -- 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
