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Subject[PATCH] loop: Do not call loop_unplug for not configured loop device.
Do not call loop_unplug for not configured loop device.

In loop_unplug() function is expected that mapping is set
and lo->lo_backing_file is not NULL.

Unfortunately loop_set_fd() set the request queue unplug function,
but loop_clr_fd() doesn't clear that.

Loop device allows open of non-configured loop in some situations.
If the unplug on request queue is called, loop module oopses because
of missing lo_backing_file.

Simple reproducer:
losetup /dev/loop0 /xxx
losetup -d /dev/loop0
dmsetup create x --table "0 1 linear /dev/loop0 0"

EIP is at loop_unplug+0x1d/0x3b
...
Call Trace:
blk_unplug+0x57/0x5e
dm_table_unplug_all+0x34/0x77 [dm_mod]
destroy_inode+0x27/0x38
generic_delete_inode+0xd5/0xd9
iput+0x4b/0x4e
dm_resume+0xca/0xfe [dm_mod]
dev_suspend+0x143/0x165 [dm_mod]
dm_ctl_ioctl+0x18e/0x1cf [dm_mod]
dev_suspend+0x0/0x165 [dm_mod]
dm_ctl_ioctl+0x0/0x1cf [dm_mod]
vfs_ioctl+0x22/0x69
do_vfs_ioctl+0x39d/0x3c7
trace_hardirqs_on+0xb/0xd
remove_vma+0x50/0x56
do_munmap+0x21c/0x237
sys_ioctl+0x2c/0x45
sysenter_do_call+0x12/0x31

Several reports here
http://www.kerneloops.org/search.php?search=loop_unplug

Fix it by simply clear unplug function together with
removing of backing file.

Signed-off-by: Milan Broz <mbroz@redhat.com>
---
drivers/block/loop.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 90d19df..1e4b41e 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -919,6 +919,7 @@ static int loop_clr_fd(struct loop_device *lo, struct block_device *bdev)

kthread_stop(lo->lo_thread);

+ lo->lo_queue->unplug_fn = NULL;
lo->lo_backing_file = NULL;

loop_release_xfer(lo);



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