On Wed, Mar 26, 2014 at 02:11:26PM -0400, Jeff Mahoney wrote:
> When encountering memory pressure, testers have run into the following
> lockdep warning. It was caused by __link_block_group calling kobject_add
> with the groups_sem held. kobject_add calls kvasprintf with GFP_KERNEL,
> which gets us into reclaim context. The kobject doesn't actually need
> to be added under the lock -- it just needs to ensure that it's only
> added for the first block group to be linked.
>
> =========================================================
> [ INFO: possible irq lock inversion dependency detected ]
> 3.14.0-rc8-default #1 Not tainted
> ---------------------------------------------------------
> kswapd0/169 just changed the state of lock:
> (&delayed_node->mutex){+.+.-.}, at: [<ffffffffa018baea>] __btrfs_release_delayed_node+0x3a/0x200 [btrfs]
> but this lock took another, RECLAIM_FS-unsafe lock in the past:
> (&found->groups_sem){+++++.}
>
> and interrupts could create inverse lock ordering between them.
>
> other info that might help us debug this:
> Possible interrupt unsafe locking scenario:
> CPU0 CPU1
> ---- ----
> lock(&found->groups_sem);
> local_irq_disable();
> lock(&delayed_node->mutex);
> lock(&found->groups_sem);
> <Interrupt>
> lock(&delayed_node->mutex);
>
> *** DEADLOCK ***
> 2 locks held by kswapd0/169:
> #0: (shrinker_rwsem){++++..}, at: [<ffffffff81159e8a>] shrink_slab+0x3a/0x160
> #1: (&type->s_umount_key#27){++++..}, at: [<ffffffff811bac6f>] grab_super_passive+0x3f/0x90
>
> Signed-off-by: Jeff Mahoney <jeffm@xxxxxxxx>
For the record, fixes https://bugzilla.kernel.org/show_bug.cgi?id=72501
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