Re: [PATCH 2/2] dm: stay in blk_queue_bypass until queue becomes initialized

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Oct 25, 2012 at 06:41:11PM +0900, Jun'ichi Nomura wrote:
> [PATCH] dm: stay in blk_queue_bypass until queue becomes initialized
> 
> With 749fefe677 ("block: lift the initial queue bypass mode on
> blk_register_queue() instead of blk_init_allocated_queue()"),
> add_disk() eventually calls blk_queue_bypass_end().
> This change invokes the following warning when multipath is used.
> 
>   BUG: scheduling while atomic: multipath/2460/0x00000002
>   1 lock held by multipath/2460:
>    #0:  (&md->type_lock){......}, at: [<ffffffffa019fb05>] dm_lock_md_type+0x17/0x19 [dm_mod]
>   Modules linked in: ...
>   Pid: 2460, comm: multipath Tainted: G        W    3.7.0-rc2 #1
>   Call Trace:
>    [<ffffffff810723ae>] __schedule_bug+0x6a/0x78
>    [<ffffffff81428ba2>] __schedule+0xb4/0x5e0
>    [<ffffffff814291e6>] schedule+0x64/0x66
>    [<ffffffff8142773a>] schedule_timeout+0x39/0xf8
>    [<ffffffff8108ad5f>] ? put_lock_stats+0xe/0x29
>    [<ffffffff8108ae30>] ? lock_release_holdtime+0xb6/0xbb
>    [<ffffffff814289e3>] wait_for_common+0x9d/0xee
>    [<ffffffff8107526c>] ? try_to_wake_up+0x206/0x206
>    [<ffffffff810c0eb8>] ? kfree_call_rcu+0x1c/0x1c
>    [<ffffffff81428aec>] wait_for_completion+0x1d/0x1f
>    [<ffffffff810611f9>] wait_rcu_gp+0x5d/0x7a
>    [<ffffffff81061216>] ? wait_rcu_gp+0x7a/0x7a
>    [<ffffffff8106fb18>] ? complete+0x21/0x53
>    [<ffffffff810c0556>] synchronize_rcu+0x1e/0x20
>    [<ffffffff811dd903>] blk_queue_bypass_start+0x5d/0x62
>    [<ffffffff811ee109>] blkcg_activate_policy+0x73/0x270
>    [<ffffffff81130521>] ? kmem_cache_alloc_node_trace+0xc7/0x108
>    [<ffffffff811f04b3>] cfq_init_queue+0x80/0x28e
>    [<ffffffffa01a1600>] ? dm_blk_ioctl+0xa7/0xa7 [dm_mod]
>    [<ffffffff811d8c41>] elevator_init+0xe1/0x115
>    [<ffffffff811e229f>] ? blk_queue_make_request+0x54/0x59
>    [<ffffffff811dd743>] blk_init_allocated_queue+0x8c/0x9e
>    [<ffffffffa019ffcd>] dm_setup_md_queue+0x36/0xaa [dm_mod]
>    [<ffffffffa01a60e6>] table_load+0x1bd/0x2c8 [dm_mod]
>    [<ffffffffa01a7026>] ctl_ioctl+0x1d6/0x236 [dm_mod]
>    [<ffffffffa01a5f29>] ? table_clear+0xaa/0xaa [dm_mod]
>    [<ffffffffa01a7099>] dm_ctl_ioctl+0x13/0x17 [dm_mod]
>    [<ffffffff811479fc>] do_vfs_ioctl+0x3fb/0x441
>    [<ffffffff811b643c>] ? file_has_perm+0x8a/0x99
>    [<ffffffff81147aa0>] sys_ioctl+0x5e/0x82
>    [<ffffffff812010be>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>    [<ffffffff814310d9>] system_call_fastpath+0x16/0x1b
> 
> The warning means during queue initialization blk_queue_bypass_start()
> calls sleeping function (synchronize_rcu) while dm holds md->type_lock.

md->type_lock is a mutex, isn't it? I thought we are allowed to block
and schedule out under mutex?

add_disk() also calls disk_alloc_events() which does kzalloc(GFP_KERNEL).
So we already have code which can block/wait under md->type_lock. I am
not sure why should we get this warning under a mutex.

Thanks
Vivek