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Subject[PATCH 1/2] bdi: Fix hung task on sync
Date
From: Derek Basehore <dbasehore@chromium.org>

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
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.

Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
CC: stable@vger.kernel.org
Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
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 e0259a163f98..8277b76be983 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1047,10 +1047,10 @@ 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 ce682f7a4f29..fab8401fc54e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -288,13 +288,16 @@ 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);
}

/*
--
1.8.1.4


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