[3.11.y.z extended stable] Patch "backing_dev: fix hung task on sync" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    backing_dev: fix hung task on sync

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 63f455b2b8410deb4c43def69ff885c6fad90cea Mon Sep 17 00:00:00 2001
From: Derek Basehore <dbasehore at chromium.org>
Date: Thu, 3 Apr 2014 14:46:22 -0700
Subject: backing_dev: fix hung task on sync

commit 6ca738d60c563d5c6cf6253ee4b8e76fa77b2b9e upstream.

bdi_wakeup_thread_delayed() used the mod_delayed_work() function 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 caused by commit
839a8e8660b6 ("writeback: replace custom worker pool implementation with
unbound workqueue").

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.

We fix the problem by using queue_delayed_work() when we want to
schedule writeback sometime in future.  This function doesn't change the
timer if it is already armed.

For the same reason, we also change 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.

[jack at suse.cz: update changelog, add comment, use bdi_wakeup_thread_delayed()]
Signed-off-by: Derek Basehore <dbasehore at chromium.org>
Reviewed-by: Jan Kara <jack at suse.cz>
Cc: Alexander Viro <viro at zento.linux.org.uk>
Reviewed-by: Tejun Heo <tj at kernel.org>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: "Darrick J. Wong" <darrick.wong at oracle.com>
Cc: Derek Basehore <dbasehore at chromium.org>
Cc: Kees Cook <keescook at chromium.org>
Cc: Benson Leung <bleung at chromium.org>
Cc: Sonny Rao <sonnyrao at chromium.org>
Cc: Luigi Semenzato <semenzato at chromium.org>
Cc: Jens Axboe <axboe at kernel.dk>
Cc: Dave Chinner <david at fromorbit.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/fs-writeback.c | 8 ++++----
 mm/backing-dev.c  | 5 ++++-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 52e5934..160f8e6 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1036,10 +1036,10 @@ void bdi_writeback_workfn(struct work_struct *work)
 		trace_writeback_pages_written(pages_written);
 	}

-	if (!list_empty(&bdi->work_list) ||
-	    (wb_has_dirty_io(wb) && dirty_writeback_interval))
-		queue_delayed_work(bdi_wq, &wb->dwork,
-			msecs_to_jiffies(dirty_writeback_interval * 10));
+	if (!list_empty(&bdi->work_list))
+		mod_delayed_work(bdi_wq, &wb->dwork, 0);
+	else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
+		bdi_wakeup_thread_delayed(bdi);

 	current->flags &= ~PF_SWAPWRITE;
 }
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e04454c..4f81435e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -285,13 +285,16 @@ int bdi_has_dirty_io(struct backing_dev_info *bdi)
  * Note, we wouldn't bother setting up the timer, but this function is on the
  * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
  * by delaying the wake-up.
+ *
+ * We have to be careful not to postpone flush work if it is scheduled for
+ * earlier. Thus we use queue_delayed_work().
  */
 void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
 {
 	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);
 }

 /*
--
1.9.1