[dm-devel] corruption causing crash in __queue_work

[Nikolay Borisov]

On Mon, Dec 14, 2015 at 5:31 PM, Mike Snitzer <snitzer at redhat.com> wrote:
> On Mon, Dec 14 2015 at  3:41P -0500,
> Nikolay Borisov <kernel at kyup.com> wrote:
>
>> Had another poke at the backtrace that is produced and here what the
>> delayed_work looks like:
>>
>> crash> struct delayed_work ffff88036772c8c0
>> struct delayed_work {
>>   work = {
>>     data = {
>>       counter = 1537
>>     },
>>     entry = {
>>       next = 0xffff88036772c8c8,
>>       prev = 0xffff88036772c8c8
>>     },
>>     func = 0xffffffffa0211a30 <do_waker>
>>   },
>>   timer = {
>>     entry = {
>>       next = 0x0,
>>       prev = 0xdead000000200200
>>     },
>>     expires = 4349463655,
>>     base = 0xffff88047fd2d602,
>>     function = 0xffffffff8106da40 <delayed_work_timer_fn>,
>>     data = 18446612146934696128,
>>     slack = -1,
>>     start_pid = -1,
>>     start_site = 0x0,
>>     start_comm =
>> "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
>>   },
>>   wq = 0xffff88030cf65400,
>>   cpu = 21
>> }
>>
>> From this it seems that the timer is also cancelled/expired judging by
>> the values in timer -> entry. But then again in dm-thin the pool is
>> first suspended, which implies the following functions were called:
>>
>> cancel_delayed_work(&pool->waker);
>> cancel_delayed_work(&pool->no_space_timeout);
>> flush_workqueue(pool->wq);
>>
>> so at that point dm-thin's workqueue should be empty and it shouldn't be
>> possible to queue any more delayed work. But the crashdump clearly shows
>> that the opposite is happening. So far all of this points to a race
>> condition and inserting some sleeps after umount and after vgchange -Kan
>> (command to disable volume group and suspend, so the cancel_delayed_work
>> is invoked) seems to reduce the frequency of crashes, though it doesn't
>> eliminate them.
>
> 'vgchange -Kan' doesn't suspend the pool before it destroys the device.
> So the cancel_delayed_work()s you referenced aren't applicable.

Hm, but does it not in fact destroy it. Using the following simple
stap script proves so:


probe module("dm_thin_pool").function("__pool_destroy") {
    print("=========__pool_destroy======");
    print_backtrace();

}

probe module("dm_thin_pool").function("pool_postsuspend") {

    printf("==== POOL_POSTSUSPEND =====\n");
    print_backtrace();

}

Produces the following backtraces:

==== POOL_POSTSUSPEND =====
 0xffffffffa033ad40 : pool_postsuspend+0x0/0x50 [dm_thin_pool]
 0xffffffff8148a5bf : suspend_targets+0x3f/0x90 [kernel]
 0xffffffff8148a668 : dm_table_postsuspend_targets+0x18/0x20 [kernel]
 0xffffffff814886dc : __dm_destroy+0x17c/0x190 [kernel]
 0xffffffff81488723 : dm_destroy+0x13/0x20 [kernel]
 0xffffffff8148f55a : dev_remove+0xfa/0x130 [kernel]
 0xffffffff8148fe94 : ctl_ioctl+0x1d4/0x2e0 [kernel]
 0xffffffff8148ffb3 : dm_ctl_ioctl+0x13/0x20 [kernel]
 0xffffffff811af3f3 : do_vfs_ioctl+0x73/0x380 [kernel]
 0xffffffff811af792 : sys_ioctl+0x92/0xa0 [kernel]
 0xffffffff8159ae2e : entry_SYSCALL_64_fastpath+0x12/0x71 [kernel]
=========__pool_destroy====== 0xffffffffa033ae20 :
__pool_destroy+0x0/0x110 [dm_thin_pool]
 0xffffffffa033af61 : __pool_dec+0x31/0x50 [dm_thin_pool]
 0xffffffffa033afae : pool_dtr+0x2e/0x70 [dm_thin_pool]
 0xffffffff8148c085 : dm_table_destroy+0x65/0x120 [kernel]
 0xffffffff8148868a : __dm_destroy+0x12a/0x190 [kernel]
 0xffffffff81488723 : dm_destroy+0x13/0x20 [kernel]
 0xffffffff8148f55a : dev_remove+0xfa/0x130 [kernel]
 0xffffffff8148fe94 : ctl_ioctl+0x1d4/0x2e0 [kernel]
 0xffffffff8148ffb3 : dm_ctl_ioctl+0x13/0x20 [kernel]
 0xffffffff811af3f3 : do_vfs_ioctl+0x73/0x380 [kernel]
 0xffffffff811af792 : sys_ioctl+0x92/0xa0 [kernel]
 0xffffffff8159ae2e : entry_SYSCALL_64_fastpath+0x12/0x71 [kernel]

When I run vgchange -Kan on a volume group. So in __dm_destroy before
dm_table_destroy (which calls pool_dtr)
the device is checked to see if it is suspended, and if not not dm
core would invoke the pre/post suspend hooks, and
this should cause the workqueue to be flushed and in quiescent state. No?

What am I missing?

>
> Can you try this patch?

I've scheduled some machines to go online with this patch and
will report back if it changes the situation. Thanks a lot!

>
> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> index 63903a5..b201d887 100644
> --- a/drivers/md/dm-thin.c
> +++ b/drivers/md/dm-thin.c
> @@ -2750,8 +2750,11 @@ static void __pool_destroy(struct pool *pool)
>         dm_bio_prison_destroy(pool->prison);
>         dm_kcopyd_client_destroy(pool->copier);
>
> -       if (pool->wq)
> +       if (pool->wq) {
> +               cancel_delayed_work(&pool->waker);
> +               cancel_delayed_work(&pool->no_space_timeout);
>                 destroy_workqueue(pool->wq);
> +       }
>
>         if (pool->next_mapping)
>                 mempool_free(pool->next_mapping, pool->mapping_pool);