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

["Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>]

On 10/27/12 05:21, Vivek Goyal wrote:
> 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?

Hm, you are right. It's a mutex.
The warning occurs only if I turned on CONFIG_PREEMPT=y.

> 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.

add_disk() is called without md->type_lock.

Call flow is like this:

dm_create
  alloc_dev
    blk_alloc_queue
    alloc_disk
    add_disk
      blk_queue_bypass_end [with 3.7-rc2]

table_load
  dm_lock_md_type [takes md->type_lock]
  dm_setup_md_queue
    blk_init_allocated_queue [when DM_TYPE_REQUEST_BASED]
      elevator_init
        blkcg_activate_policy
          blk_queue_bypass_start <-- THIS triggers the warning
          blk_queue_bypass_end
      blk_queue_bypass_end [with 3.6]
  dm_unlock_md_type

blk_queue_bypass_start() in blkcg_activate_policy was nested call,
that did nothing, with 3.6.
With 3.7-rc2, it becomes the initial call and does
actual draining stuff.

-- 
Jun'ichi Nomura, NEC Corporation