1. 15 Apr, 2016 1 commit
    • Gerald Schaefer's avatar
      s390/dcssblk: fix possible deadlock in remove vs. per-device attributes · 1378a683
      Gerald Schaefer authored
      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: default avatarGerald Schaefer <gerald.schaefer@de.ibm.com>
      Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
      1378a683
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