On Wed, Jun 05, 2013 at 08:38:59AM -0400, Jeff Layton wrote: > On Wed, 5 Jun 2013 08:24:32 -0400 > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > On Wed, Jun 05, 2013 at 07:38:22AM -0400, Jeff Layton wrote: > > > On Tue, 4 Jun 2013 17:58:39 -0400 > > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > > > > > On Fri, May 31, 2013 at 11:07:30PM -0400, Jeff Layton wrote: > > > > > Currently, when there is a lot of lock contention the kernel spends an > > > > > inordinate amount of time taking blocked locks off of the global > > > > > blocked_list and then putting them right back on again. When all of this > > > > > code was protected by a single lock, then it didn't matter much, but now > > > > > it means a lot of file_lock_lock thrashing. > > > > > > > > > > Optimize this a bit by deferring the removal from the blocked_list until > > > > > we're either applying or cancelling the lock. By doing this, and using a > > > > > lockless list_empty check, we can avoid taking the file_lock_lock in > > > > > many cases. > > > > > > > > > > Because the fl_link check is lockless, we must ensure that only the task > > > > > that "owns" the request manipulates the fl_link. Also, with this change, > > > > > it's possible that we'll see an entry on the blocked_list that has a > > > > > NULL fl_next pointer. In that event, just ignore it and continue walking > > > > > the list. > > > > > > > > OK, that sounds safe as in it shouldn't crash, but does the deadlock > > > > detection still work, or can it miss loops? > > > > > > > > Those locks that are temporarily NULL would previously not have been on > > > > the list at all, OK, but... I'm having trouble reasoning about how this > > > > works now. > > > > > > > > Previously a single lock was held interrupted across > > > > posix_locks_deadlock and locks_insert_block() which guaranteed we > > > > shouldn't be adding a loop, is that still true? > > > > > > > > --b. > > > > > > > > > > I had thought it was when I originally looked at this, but now that I > > > consider it again I think you may be correct and that there are possible > > > races here. Since we might end up reblocking behind a different lock > > > without taking the global spinlock we could flip to blocking behind a > > > different lock such that a loop is created if you had a complex (>2) > > > chain of locks. > > > > > > I think I'm going to have to drop this approach and instead make it so > > > that the deadlock detection and insertion into the global blocker > > > list/hash are atomic. > > > > Right. Once you drop the lock you can no longer be sure that what you > > learned about the file-lock graph stays true. > > > > > Ditto for locks_wake_up_blocks on posix locks and > > > taking the entries off the list/hash. > > > > Here I'm not sure what you mean. > > > > Basically, I mean that rather than setting the fl_next pointer to NULL > while holding only the inode lock and then ignoring those locks in the > deadlock detection code, we should additionally take the global lock in > locks_wake_up_blocks too and take the blocked locks off the global list > and the i_flock list at the same time. OK, thanks, got it. I have a hard time thinking about that.... But yes it bothers me that the deadlock detection code could see an out-of-date value of fl_next, and I can't convince myself that this wouldn't result in false positives or false negatives. > That actually might not be completely necessary, but it'll make the > logic clearer and easier to understand and probably won't hurt > performance too much. Again, I'll need to do some perf testing to be > sure. OK! --b. -- 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
