On Wed, 19 Feb 2014 14:01:39 -0500 Tejun Heo <[email protected]> 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 <[email protected]> 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 <[email protected]> Reviewed-by: Jan Kara <[email protected]> Cc: Alexander Viro <[email protected]> Cc: Tejun Heo <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: "Darrick J. Wong" <[email protected]> Cc: Derek Basehore <[email protected]> Cc: Kees Cook <[email protected]> Cc: Benson Leung <[email protected]> Cc: Sonny Rao <[email protected]> Cc: Luigi Semenzato <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Dave Chinner <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
