linux-kernel - BUG: lock held when returning to user space

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Date:	Wed, 12 Mar 2008 16:45:36 +0100
From:	Frank Munzert <frankm@...ux.vnet.ibm.com>
To:	mingo@...e.hu
CC:	linux-kernel@...r.kernel.org
Subject: BUG: lock held when returning to user space

Hi Ingo,

we provided a device driver vmur dealing with z/VM virtual unit record 
devices (reader, punch, printer). A corresponding user space tool 
provides functions similar to the CMS commands RECEIVE, PUNCH, PRINT.  
Unit record devices are not meant for concurrent read or write by 
multiple users, that's why we need to serialize access. The driver's 
open method uses  mutex_trylock or  mutex_lock_interruptible  to  ensure 
exclusive access to the device, while its  release method  uses 
mutex_unlock.

As a consequence, lockdep complains about locks being held when 
returning to user space. We used a very simple char device driver 
(appended below) to produce this message: 
================================================
[ BUG: lock held when returning to user space! ]
------------------------------------------------
testapp/2683 is leaving the kernel with locks still held!
1 lock held by testapp/2683:
 #0:  (&test_mutex){--..}, at: [<000003e00003316c>] test_open+0x30/0x64 
[test]

For the vmur device driver it is crucial to have only one process access 
a given unit record device node at a given time. So having open hold the 
mutex and return to user space is exactly what we want. Is  there any 
annotation  to tell lockdep to suppress or bypass this kind of warning?

Thanks in advance,

Frank

View attachment "test.c" of type "text/plain" (1931 bytes)