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[AX25] af_ax25: Possible circular locking.

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Bernard Pidoux F6BVP reported:
> When I killall kissattach I can see the following message.
>
> This happens on kernel 2.6.24-rc5 already patched with the 6 previously
> patches I sent recently.
>
>
> =======================================================
> [ INFO: possible circular locking dependency detected ]
> 2.6.23.9 #1
> -------------------------------------------------------
> kissattach/2906 is trying to acquire lock:
>  (linkfail_lock){-+..}, at: [<d8bd4603>] ax25_link_failed+0x11/0x39 [ax25]
>
> but task is already holding lock:
>  (ax25_list_lock){-+..}, at: [<d8bd7c7c>] ax25_device_event+0x38/0x84
> [ax25]
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
...

lockdep is worried about the different order here:

#1 (rose_neigh_list_lock){-+..}:
#3 (ax25_list_lock){-+..}:

#0 (linkfail_lock){-+..}:
#1 (rose_neigh_list_lock){-+..}:

#3 (ax25_list_lock){-+..}:
#0 (linkfail_lock){-+..}:

So, ax25_list_lock could be taken before and after linkfail_lock. 
I don't know if this three-thread clutch is very probable (or
possible at all), but it seems another bug reported by Bernard
("[...] system impossible to reboot with linux-2.6.24-rc5")
could have similar source - namely ax25_list_lock held by
ax25_kill_by_device() during ax25_disconnect(). It looks like the
only place which calls ax25_disconnect() this way, so I guess, it
isn't necessary.

This patch is breaking the lock for ax25_disconnect().

Reported-and-tested-by: Bernard Pidoux <f6bvp@free.fr>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jarek Poplawski 2008-01-10 21:21:20 -08:00 committed by David S. Miller
parent 27d1cba21f
commit ecd2ebdea3
1 changed files with 12 additions and 0 deletions
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12
net/ax25/af_ax25.c
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@ -87,10 +87,22 @@ static void ax25_kill_by_device(struct net_device *dev)
return;
spin_lock_bh(&ax25_list_lock);
again:
ax25_for_each(s, node, &ax25_list) {
if (s->ax25_dev == ax25_dev) {
s->ax25_dev = NULL;
spin_unlock_bh(&ax25_list_lock);
ax25_disconnect(s, ENETUNREACH);
spin_lock_bh(&ax25_list_lock);
/* The entry could have been deleted from the
* list meanwhile and thus the next pointer is
* no longer valid. Play it safe and restart
* the scan. Forward progress is ensured
* because we set s->ax25_dev to NULL and we
* are never passed a NULL 'dev' argument.
*/
goto again;
}
}
spin_unlock_bh(&ax25_list_lock);