s390/dcssblk: fix possible deadlock in remove vs. per-device attributes
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>
diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
index 1bce9cf..b839086 100644
--- a/drivers/s390/block/dcssblk.c
+++ b/drivers/s390/block/dcssblk.c
@@ -756,15 +756,16 @@
blk_cleanup_queue(dev_info->dcssblk_queue);
dev_info->gd->queue = NULL;
put_disk(dev_info->gd);
- device_unregister(&dev_info->dev);
/* unload all related segments */
list_for_each_entry(entry, &dev_info->seg_list, lh)
segment_unload(entry->segment_name);
- put_device(&dev_info->dev);
up_write(&dcssblk_devices_sem);
+ device_unregister(&dev_info->dev);
+ put_device(&dev_info->dev);
+
rc = count;
out_buf:
kfree(local_buf);