[dm-devel] [PATCH 2.6.20-rc5] dm-multipath: fix stall on noflush suspend/resume

[Jun'ichi Nomura]

Allow noflush suspend/resume of device-mapper device only for
the case where the device size is unchanged.

Otherwise, dm-multipath devices can stall when resumed if noflush
was used when suspending them, all paths have failed and
queue_if_no_path is set.

Explanation:
 1. Something is doing fsync() on the block dev,
    holding inode->i_sem
 2. The fsync write is blocked by all-paths-down and queue_if_no_path
 3. Someone requests to suspend the dm device with noflush.
    Pending writes are left in queue.
 4. In the middle of dm_resume(), __bind() tries to get
    inode->i_sem to do __set_size() and waits forever.

Signed-off-by: Jun'ichi Nomura <j-nomura at ce.jp.nec.com>

---
'noflush suspend' is a new device-mapper feature introduced in
early 2.6.20. So I hope the fix being included before 2.6.20 is
released.

Example of reproducer:
 1. Create a multipath device by dmsetup
 2. Fail all paths during mkfs
 3. Do dmsetup suspend --noflush and load new map with healthy paths
 4. Do dmsetup resume


 drivers/md/dm.c |   27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

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