[PATCH v3] loop: fix I/O error on fsync() in detached loop devices

[Mauricio Faria de Oliveira <mfo@canonical.com>]

There's an I/O error on fsync() in a detached loop device
if it has been previously attached.

The issue is write cache is enabled in the attach path in
loop_configure() but it isn't disabled in the detach path;
thus it remains enabled in the block device regardless of
whether it is attached or not.

Now fsync() can get an I/O request that will just be failed
later in loop_queue_rq() as device's state is not 'Lo_bound'.

So, disable write cache in the detach path.

Do so based on the queue flag, not the loop device flag for
read-only (used to enable) as the queue flag can be changed
via sysfs even on read-only loop devices (e.g., losetup -r.)

Test-case:

    # DEV=/dev/loop7

    # IMG=/tmp/image
    # truncate --size 1M $IMG

    # losetup $DEV $IMG
    # losetup -d $DEV

Before:

    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = -1 EIO (Input/output error)
    Warning: Error fsyncing/closing /dev/loop7: Input/output error
    [  982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0

After:

    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = 0

Co-developed-by: Eric Desrochers <eric.desrochers@canonical.com>
Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Tested-by: Gabriel Krisman Bertazi <krisman@collabora.com>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

v3:
- Disable write cache based on QUEUE_FLAG_WC, not just
  !LO_FLAGS_READ_ONLY, as the former can be enabled in
  sysfs even for LO_FLAGS_READ_ONLY, thus not disabled
  later. Mention that in commit message. (thanks, Ming)
v2:
- Fix ordering of Co-developed-by:/Signed-off-by: tags.
  (thanks, Krisman)
- Add Tested-by: tag. (likewise.)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index e5ff328f0917..e94a11dbb5bd 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1212,6 +1212,9 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 		goto out_unlock;
 	}
 
+	if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags))
+		blk_queue_write_cache(lo->lo_queue, false, false);
+
 	/* freeze request queue during the transition */
 	blk_mq_freeze_queue(lo->lo_queue);
 
-- 
2.27.0