On Fri, Oct 09, 2015 at 12:35:48AM +0000, Kosuke Tatsukawa wrote: > This patch removes the call to waitqueue_active() leaving just wake_up() > behind. This fixes the problem because the call to spin_lock_irqsave() > in wake_up() will be an ACQUIRE operation. Either we can switch it to wake_up or put the barrier before the check. Not all instances of waitqueue_active need the barrier though. > I found this issue when I was looking through the linux source code > for places calling waitqueue_active() before wake_up*(), but without > preceding memory barriers, after sending a patch to fix a similar > issue in drivers/tty/n_tty.c (Details about the original issue can be > found here: https://lkml.org/lkml/2015/9/28/849). There are more in btrfs: https://www.mail-archive.com/linux-btrfs%40vger.kernel.org/msg41914.html > @@ -918,9 +918,7 @@ void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info *fs_info) > void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount) > { > percpu_counter_sub(&fs_info->bio_counter, amount); > - > - if (waitqueue_active(&fs_info->replace_wait)) > - wake_up(&fs_info->replace_wait); > + wake_up(&fs_info->replace_wait); Chris had a comment on that one in https://www.mail-archive.com/linux-btrfs%40vger.kernel.org/msg42551.html it's in performance critial context and the explicit wake_up is even worse than the barrier. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
