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* [PATCH AUTOSEL 4.4 44/64] bdev: Reduce time holding bd_mutex in sync in blkdev_close()
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@ 2020-09-18  2:16 ` Sasha Levin
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From: Sasha Levin @ 2020-09-18  2:16 UTC (permalink / raw)
  To: linux-kernel, stable
  Cc: Douglas Anderson, Guenter Roeck, Christoph Hellwig, Jens Axboe,
	Sasha Levin, linux-fsdevel

From: Douglas Anderson <>

[ Upstream commit b849dd84b6ccfe32622988b79b7b073861fcf9f7 ]

While trying to "dd" to the block device for a USB stick, I
encountered a hung task warning (blocked for > 120 seconds).  I
managed to come up with an easy way to reproduce this on my system
(where /dev/sdb is the block device for my USB stick) with:

  while true; do dd if=/dev/zero of=/dev/sdb bs=4M; done

With my reproduction here are the relevant bits from the hung task

 INFO: task udevd:294 blocked for more than 122 seconds.
 udevd           D    0   294      1 0x00400008
 Call trace:

 Showing all locks held in the system:
 1 lock held by dd/2798:
  #0: ffffff814ac1a3b8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x50/0x204
 dd              D    0  2798   2764 0x00400208
 Call trace:

The problem appears related to the fact that my USB disk is terribly
slow and that I have a lot of RAM in my system to cache things.
Specifically my writes seem to be happening at ~15 MB/s and I've got
~4 GB of RAM in my system that can be used for buffering.  To write 4
GB of buffer to disk thus takes ~4000 MB / ~15 MB/s = ~267 seconds.

The 267 second number is a problem because in __blkdev_put() we call
sync_blockdev() while holding the bd_mutex.  Any other callers who
want the bd_mutex will be blocked for the whole time.

The problem is made worse because I believe blkdev_put() specifically
tells other tasks (namely udev) to go try to access the device at right
around the same time we're going to hold the mutex for a long time.

Putting some traces around this (after disabling the hung task detector),
I could confirm:
 dd:    437.608600: __blkdev_put() right before sync_blockdev() for sdb
 udevd: 437.623901: blkdev_open() right before blkdev_get() for sdb
 dd:    661.468451: __blkdev_put() right after sync_blockdev() for sdb
 udevd: 663.820426: blkdev_open() right after blkdev_get() for sdb

A simple fix for this is to realize that sync_blockdev() works fine if
you're not holding the mutex.  Also, it's not the end of the world if
you sync a little early (though it can have performance impacts).
Thus we can make a guess that we're going to need to do the sync and
then do it without holding the mutex.  We still do one last sync with
the mutex but it should be much, much faster.

With this, my hung task warnings for my test case are gone.

Signed-off-by: Douglas Anderson <>
Reviewed-by: Guenter Roeck <>
Reviewed-by: Christoph Hellwig <>
Signed-off-by: Jens Axboe <>
Signed-off-by: Sasha Levin <>
 fs/block_dev.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index b2ebfd96785b7..a71d442ef7d0e 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1515,6 +1515,16 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
 	struct gendisk *disk = bdev->bd_disk;
 	struct block_device *victim = NULL;
+	/*
+	 * Sync early if it looks like we're the last one.  If someone else
+	 * opens the block device between now and the decrement of bd_openers
+	 * then we did a sync that we didn't need to, but that's not the end
+	 * of the world and we want to avoid long (could be several minute)
+	 * syncs while holding the mutex.
+	 */
+	if (bdev->bd_openers == 1)
+		sync_blockdev(bdev);
 	mutex_lock_nested(&bdev->bd_mutex, for_part);
 	if (for_part)

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