[dm-devel] [PATCH] Revert "dm bufio: fix deadlock with loop device"

[Mikulas Patocka]

Revert the patch bd293d071ffe65e645b4d8104f9d8fe15ea13862. A proper fix
should be not to use GFP_KERNEL in the function do_blockdev_direct_IO.

Note that the patch bd293d071ffe doesn't really prevent the deadlock from
occuring - if we look at the stacktrace reported by Junxiao Bi, we see
that it hangs in bit_wait_io and not on the mutex - i.e. it has already
successfully taken the mutex. Changing the mutex from mutex_lock to
mutex_trylock won't help with deadlocks that happen afterwards.

PID: 474    TASK: ffff8813e11f4600  CPU: 10  COMMAND: "kswapd0"
   #0 [ffff8813dedfb938] __schedule at ffffffff8173f405
   #1 [ffff8813dedfb990] schedule at ffffffff8173fa27
   #2 [ffff8813dedfb9b0] schedule_timeout at ffffffff81742fec
   #3 [ffff8813dedfba60] io_schedule_timeout at ffffffff8173f186
   #4 [ffff8813dedfbaa0] bit_wait_io at ffffffff8174034f
   #5 [ffff8813dedfbac0] __wait_on_bit at ffffffff8173fec8
   #6 [ffff8813dedfbb10] out_of_line_wait_on_bit at ffffffff8173ff81
   #7 [ffff8813dedfbb90] __make_buffer_clean at ffffffffa038736f [dm_bufio]
   #8 [ffff8813dedfbbb0] __try_evict_buffer at ffffffffa0387bb8 [dm_bufio]
   #9 [ffff8813dedfbbd0] dm_bufio_shrink_scan at ffffffffa0387cc3 [dm_bufio]
  #10 [ffff8813dedfbc40] shrink_slab at ffffffff811a87ce
  #11 [ffff8813dedfbd30] shrink_zone at ffffffff811ad778
  #12 [ffff8813dedfbdc0] kswapd at ffffffff811ae92f
  #13 [ffff8813dedfbec0] kthread at ffffffff810a8428
  #14 [ffff8813dedfbf50] ret_from_fork at ffffffff81745242

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Cc: stable at vger.kernel.org
Fixes: bd293d071ffe ("dm bufio: fix deadlock with loop device")

---
 drivers/md/dm-bufio.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c	2019-08-02 08:52:35.000000000 +0200
+++ linux-2.6/drivers/md/dm-bufio.c	2019-08-08 11:19:13.000000000 +0200
@@ -1604,7 +1604,9 @@ dm_bufio_shrink_scan(struct shrinker *sh
 	unsigned long freed;
 
 	c = container_of(shrink, struct dm_bufio_client, shrinker);
-	if (!dm_bufio_trylock(c))
+	if (sc->gfp_mask & __GFP_FS)
+		dm_bufio_lock(c);
+	else if (!dm_bufio_trylock(c))
 		return SHRINK_STOP;
 
 	freed  = __scan(c, sc->nr_to_scan, sc->gfp_mask);