[dm-devel] corruption causing crash in __queue_work

[Mike Snitzer]

On Mon, Dec 14 2015 at  3:11pm -0500,
Nikolay Borisov <kernel at kyup.com> wrote:

> 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?

Nothing, clearly you're right!
 
> >
> > 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!

Shouldn't make any difference given the above.

But in that the suspend hooks are used during destroy (if not already
suspended): makes this report all the more bizarre.