On Wed, 19 Feb 2014 14:01:39 -0500 Tejun Heo <tj@xxxxxxxxxx> wrote: > Hello, Jan. > > On Wed, Feb 19, 2014 at 10:27:31AM +0100, Jan Kara wrote: > > You are the workqueue expert so you may know better ;) But the way I > > understand it is that queue_delayed_work() does nothing if the timer is > > already running. Since we queue flusher work to run either immediately or > > after dirty_writeback_interval we are safe to run queue_delayed_work() > > whenever we want it to run after dirty_writeback_interval and > > mod_delayed_work() whenever we want to run it immediately. > > Ah, okay, so it's always mod on immediate and queue on delayed. Yeah, > that should work. > > > But it's subtle and some interface where we could say queue delayed work > > after no later than X would be easier to grasp. > > Yeah, I think it'd be better if we had something like > mod_delayed_work_if_later(). Hmm... The code comments which you asked for were not forthcoming. Are you otherwise OK with merging this into 3.14 and -stable? From: Derek Basehore <dbasehore@xxxxxxxxxxxx> Subject: backing_dev: fix hung task on sync bdi_wakeup_thread_delayed() used mod_delayed_work() to schedule work to writeback dirty inodes. The problem with this is that it can delay work that is scheduled for immediate execution, such as the work from sync_inodes_sb(). This can happen since mod_delayed_work can now steal work from a work_queue. This fixes the problem by using queue_delayed_work instead. This is a regression from the move to the bdi workqueue design. The reason that this causes a problem is that laptop-mode will change the delay, dirty_writeback_centisecs, to 60000 (10 minutes) by default. In the case that bdi_wakeup_thread_delayed races with sync_inodes_sb, sync will be stopped for 10 minutes and trigger a hung task. Even if dirty_writeback_centisecs is not long enough to cause a hung task, we still don't want to delay sync for that long. For the same reason, this also changes bdi_writeback_workfn to immediately queue the work again in the case that the work_list is not empty. The same problem can happen if the sync work is run on the rescue worker. Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx> Reviewed-by: Jan Kara <jack@xxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx> Cc: Derek Basehore <dbasehore@xxxxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Benson Leung <bleung@xxxxxxxxxxxx> Cc: Sonny Rao <sonnyrao@xxxxxxxxxxxx> Cc: Luigi Semenzato <semenzato@xxxxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Cc: Dave Chinner <david@xxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/fs-writeback.c | 5 +++-- mm/backing-dev.c | 2 +- 2 files changed, 4 insertions(+), 3 deletions(-) diff -puN fs/fs-writeback.c~backing_dev-fix-hung-task-on-sync fs/fs-writeback.c --- a/fs/fs-writeback.c~backing_dev-fix-hung-task-on-sync +++ a/fs/fs-writeback.c @@ -1039,8 +1039,9 @@ void bdi_writeback_workfn(struct work_st trace_writeback_pages_written(pages_written); } - if (!list_empty(&bdi->work_list) || - (wb_has_dirty_io(wb) && dirty_writeback_interval)) + if (!list_empty(&bdi->work_list)) + mod_delayed_work(bdi_wq, &wb->dwork, 0); + else if (wb_has_dirty_io(wb) && dirty_writeback_interval) queue_delayed_work(bdi_wq, &wb->dwork, msecs_to_jiffies(dirty_writeback_interval * 10)); diff -puN mm/backing-dev.c~backing_dev-fix-hung-task-on-sync mm/backing-dev.c --- a/mm/backing-dev.c~backing_dev-fix-hung-task-on-sync +++ a/mm/backing-dev.c @@ -294,7 +294,7 @@ void bdi_wakeup_thread_delayed(struct ba unsigned long timeout; timeout = msecs_to_jiffies(dirty_writeback_interval * 10); - mod_delayed_work(bdi_wq, &bdi->wb.dwork, timeout); + queue_delayed_work(bdi_wq, &bdi->wb.dwork, timeout); } /* _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>