Lockdep violation in pcmcia

I've been getting these warnings for a long, long time, and finally
decided to report them:

[ 1893.033051] =============================================
[ 1893.036023] [ INFO: possible recursive locking detected ]
[ 1893.036023] 2.6.31 #2
[ 1893.036023] ---------------------------------------------
[ 1893.036023] cardmgr/1457 is trying to acquire lock:
[ 1893.036023] (pcmcia_socket_list_rwsem){++++.+}, at: [<c8874867>] adjust_memory+0x71/0xbd [rsrc_nonstatic]
[ 1893.036023]
[ 1893.036023] but task is already holding lock:
[ 1893.036023] (pcmcia_socket_list_rwsem){++++.+}, at: [<c8972e5b>] ds_ioctl+0x282/0xa6b [pcmcia]
[ 1893.036023]
[ 1893.036023] other info that might help us debug this:
[ 1893.036023] 2 locks held by cardmgr/1457:
[ 1893.036023] #0: (pcmcia_socket_list_rwsem){++++.+}, at: [<c8972e5b>] ds_ioctl+0x282/0xa6b [pcmcia]
[ 1893.036023] #1: (rsrc_mutex){+.+.+.}, at: [<c887482f>] adjust_memory+0x39/0xbd [rsrc_nonstatic]
[ 1893.036023]
[ 1893.036023] stack backtrace:
[ 1893.036023] Pid: 1457, comm: cardmgr Not tainted 2.6.31 #2
[ 1893.036023] Call Trace:
[ 1893.036023] [<c1040143>] __lock_acquire+0x741/0xa61
[ 1893.036023] [<c103ed06>] ? mark_held_locks+0x3b/0x56
[ 1893.036023] [<c10404bb>] lock_acquire+0x58/0x7a
[ 1893.036023] [<c8874867>] ? adjust_memory+0x71/0xbd [rsrc_nonstatic]
[ 1893.036023] [<c11c1f28>] down_read+0x2a/0x3e
[ 1893.036023] [<c8874867>] ? adjust_memory+0x71/0xbd [rsrc_nonstatic]
[ 1893.036023] [<c8874867>] adjust_memory+0x71/0xbd [rsrc_nonstatic]
[ 1893.036023] [<c88747f6>] ? adjust_memory+0x0/0xbd [rsrc_nonstatic]
[ 1893.036023] [<c8972f05>] ds_ioctl+0x32c/0xa6b [pcmcia]
[ 1893.036023] [<c10efa65>] ? _raw_spin_trylock+0x2b/0x30
[ 1893.036023] [<c107c077>] vfs_ioctl+0x4c/0x65
[ 1893.036023] [<c107c58a>] do_vfs_ioctl+0x451/0x478
[ 1893.036023] [<c1040987>] ? lock_release+0x12c/0x133
[ 1893.036023] [<c107c5db>] sys_ioctl+0x2a/0x43
[ 1893.036023] [<c1002a48>] sysenter_do_call+0x12/0x36

Is this problem already well known?

The cause is easy enough to track down. In pcmcia_ioctl.c,
pcmcia_adjust_resource_info() does a down_read() on
pcmcia_socket_list_rwsem. While holding the rwsem, one of the pathways
calls the s->resource_ops->add_mem method. On my system this method is
realized by adjust_memory() in rsrc_nonstatic.c, which does its own
down_read() on the same rwsem -- i.e., a recursive locking attempt.

The reason lockdep warns about this behavior is that it can lead to
deadlock in the wrong circumstances, namely, if another thread were to
do a down_write() in between the two down_read() calls.

Would it be correct simply to omit the down_read() in adjust_memory()?

Alan Stern

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Hey,

On Mon, Nov 02, 2009 at 04:21:23PM -0500, Alan Stern wrote:
> I've been getting these warnings for a long, long time, and finally
> decided to report them:

Thanks!

> [ 1893.036023] other info that might help us debug this:
> [ 1893.036023] 2 locks held by cardmgr/1457:

Wow, cardmgr still in use... I had hoped it had already disappeared on
non-embedded systems...

> The cause is easy enough to track down. In pcmcia_ioctl.c,
> pcmcia_adjust_resource_info() does a down_read() on
> pcmcia_socket_list_rwsem. While holding the rwsem, one of the pathways
> calls the s->resource_ops->add_mem method. On my system this method is
> realized by adjust_memory() in rsrc_nonstatic.c, which does its own
> down_read() on the same rwsem -- i.e., a recursive locking attempt.
>
> The reason lockdep warns about this behavior is that it can lead to
> deadlock in the wrong circumstances, namely, if another thread were to
> do a down_write() in between the two down_read() calls.
>
> Would it be correct simply to omit the down_read() in adjust_memory()?

No, for there are other code paths reaching adjust_memory() not holding a
(read) lock on this semaphore. As pcmcia_ioctl.c is on its way out anyways:
would you mind keeping it as it is, for a down_write() call is quite
unlikely during the only time cardmgr actually does this call (system
startup)?

Best,
Dominik
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