[dm-devel] [PATCH 8/9] dm: Fix two race conditions related to stopping and starting queues

[Bart Van Assche]

On 09/01/2016 09:12 AM, Mike Snitzer wrote:
> On Thu, Sep 01 2016 at 11:50am -0400,
> Mike Snitzer <snitzer at redhat.com> wrote:
>
>> On Thu, Sep 01 2016 at 11:31am -0400,
>> Bart Van Assche <bart.vanassche at sandisk.com> wrote:
>>
>>> On 09/01/2016 08:05 AM, Mike Snitzer wrote:
>>>> I've staged most of your changes (with slight tweaks), see:
>>>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.9
>>>>
>>>> Only remaining issue is the queue dying race(s) in dm-multipath.
>>>
>>> Thanks Mike! Two minor comments though:
>>> * In dm_start_queue(), I think that the queue_flag_clear_unlocked()
>>>   call should be converted into queue_flag_clear() and that it should
>>>   be protected by the block layer queue lock. Every call of
>>>   queue_flag_clear_unlocked() after block device initialization has
>>>   finished is wrong if blk_cleanup_queue() can be called concurrently.
>>
>> OK, I'll have a look.
>
> Please see/test the dm-4.8 and dm-4.9 branches (dm-4.9 being rebased
> ontop of dm-4.8):
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.8
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.9

Hello Mike,

The result of my tests of the dm-4.9 branch is as follows:
* With patch "dm mpath: check if path's request_queue is dying in 
activate_path()" I still see every now and then that CPU usage of one of 
the kworker threads jumps to 100%.
* A "if (!blk_queue_stopped(q))" test needs to be added in 
dm_stop_queue() to avoid the following hang (that test was present in my 
version of the patch that adds the blk_mq_{freeze,unfreeze}_queue() calls):

     sysrq: SysRq : Show Blocked State
       task                        PC stack   pid father
     multipathd      D ffff8803c8d37b80     0  3242      1 0x00000000
     Call Trace:
      [<ffffffff81627087>] schedule+0x37/0x90
      [<ffffffff813097e1>] blk_mq_freeze_queue_wait+0x51/0xb0
      [<ffffffff8130be05>] blk_mq_freeze_queue+0x15/0x20
      [<ffffffffa034d882>] dm_stop_queue+0x62/0xc0 [dm_mod]
      [<ffffffffa0342a1b>] dm_swap_table+0x2fb/0x370 [dm_mod]
      [<ffffffffa0347875>] dev_suspend+0x95/0x220 [dm_mod]
      [<ffffffffa03480fc>] ctl_ioctl+0x1fc/0x550 [dm_mod]
      [<ffffffffa034845e>] dm_ctl_ioctl+0xe/0x20 [dm_mod]
      [<ffffffff811ee27f>] do_vfs_ioctl+0x8f/0x690
      [<ffffffff811ee8bc>] SyS_ioctl+0x3c/0x70
      [<ffffffff8162d125>] entry_SYSCALL_64_fastpath+0x18/0xa8

Thanks,

Bart.