dcssblk_remove_store() holds the dcssblk_devices_sem semaphore while
calling device_unregister(), which in turn tries to acquire the kernfs
kn->dev_map rwsem for the device sysfs subtree. The same rwsem is also
acquired when using the per-device sysfs attributes in the device sub-tree,
and the attribute handlers then also acquire the dcssblk_devices_sem.
This can lead to a deadlock when removing a DCSS while concurrently
reading from / writing to one of its sysfs attributes. The following
lockdep warning hinted towards the issue (CPU0 = dcssblk_remove_store,
CPU1 = dcssblk_shared_store):
[ 76.496047] Possible unsafe locking scenario:
[ 76.496054] CPU0 CPU1
[ 76.496059] ---- ----
[ 76.496087] lock(&dcssblk_devices_sem);
[ 76.496090] lock(s_active#175);
[ 76.496106] lock(&dcssblk_devices_sem);
[ 76.496110] lock(s_active#175);
[ 76.496115]
*** DEADLOCK ***
Fix this by releasing the dcssblk_devices_sem semaphore, which only
protects internal DCSS data, before calling device_unregister().
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
1378a68344