On Mon, Oct 17 2016 at  9:16pm -0400,
Tahsin Erdogan [off-list ref] wrote:

cleanup_mapped_device() calls kthread_stop() if kworker_task is
non-NULL. Currently the assigned value could be a valid task struct or
an error code. Do not assign in case of error.

Example failure when kthread_run() returns -ENOMEM:

[   22.255939] BUG: unable to handle kernel NULL pointer dereference at 000000000000000c
[   22.258847] IP: [<ffffffff802973a4>] kthread_stop+0x34/0x260
[   22.260130] PGD 78a23067 PUD 78b56067 PMD 0
[   22.260130] Oops: 0002 [#1] SMP
[   22.260130] CPU: 1 PID: 1849 Comm: dmsetup Tainted: G        W 4.8.0+ #3
[   22.260130] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   22.260130] task: ffff880078966400 task.stack: ffffc90001898000
[   22.260130] RIP: 0010:[<ffffffff802973a4>]  [<ffffffff802973a4>] kthread_stop+0x34/0x260
[   22.260130] RSP: 0018:ffffc9000189bc40  EFLAGS: 00010202
[   22.260130] RAX: 0000000000000001 RBX: fffffffffffffff4 RCX: 0000000000000003
[   22.260130] RDX: ffff88007fd18600 RSI: 0000000000000001 RDI: ffffffff81037080
[   22.260130] RBP: ffffc9000189bc50 R08: 0000000000000000 R09: 0000000000000000
[   22.260130] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[   22.260130] R13: 0000000000000001 R14: ffff880077f539d8 R15: 0000000000000004
[   22.260130] FS:  00007fc9ef2e2840(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
[   22.260130] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   22.260130] CR2: 000000000000000c CR3: 0000000077fa2000 CR4: 00000000000006e0
[   22.260130] Stack:
[   22.260130]  ffff880077f53800 0000000000000000 ffffc9000189bc68 ffffffff808b26fa
[   22.260130]  ffff880077f53800 ffffc9000189bcb0 ffffffff808b3c58 0000000000000000
[   22.260130]  00000000808b534b ffffc9000189bd20 ffff880077f53800 0000000000000000
[   22.260130] Call Trace:
[   22.260130]  [<ffffffff808b26fa>] cleanup_mapped_device+0x2a/0xe0
[   22.260130]  [<ffffffff808b3c58>] __dm_destroy+0x1a8/0x2b0
[   22.260130]  [<ffffffff808b4b6e>] dm_destroy+0xe/0x10
[   22.260130]  [<ffffffff808b9f49>] dev_remove+0xd9/0x120
[   22.260130]  [<ffffffff808b9e70>] ? dev_suspend+0x210/0x210
[   22.260130]  [<ffffffff808ba576>] ctl_ioctl+0x206/0x500
[   22.260130]  [<ffffffff808ba87e>] dm_ctl_ioctl+0xe/0x20
[   22.260130]  [<ffffffff803bca40>] do_vfs_ioctl+0x90/0x6b0
[   22.260130]  [<ffffffff80b11fd7>] ?  entry_SYSCALL_64_fastpath+0x5/0xad
[   22.260130]  [<ffffffff802bd974>] ?  trace_hardirqs_on_caller+0xf4/0x1c0
[   22.260130]  [<ffffffff803bd0d4>] SyS_ioctl+0x74/0x80
[   22.260130]  [<ffffffff80b11fea>] entry_SYSCALL_64_fastpath+0x18/0xad
[   22.260130] Code: e5 41 54 85 c0 53 48 89 fb 0f 8f bb 01 00 00 65 8b
05 a1 2d d7 7f 89 c0 48 0f a3 05 9f 94 e8 00 0f 92 c0 84 c0 0f 85 a3 00
00 00 <f0> ff 43 18 48 89 df e8 10 f8 ff ff 48 85 c0 49 89 c4 74 29 f0
[   22.260130] RIP  [<ffffffff802973a4>] kthread_stop+0x34/0x260
[   22.260130]  RSP <ffffc9000189bc40>
[   22.260130] CR2: 000000000000000c
[   22.301062] ---[ end trace 22b4f4f62c04f3cf ]---

Signed-off-by: Tahsin Erdogan <redacted>

Thanks for the patch but I elected to fix this issue a slightly
different way, please see this commit staged for 4.9-rcX:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.9&id=937fa62e8a00d0b4bc2c0a40567d7c88ab2b2e8d

(also, your mail reminds me that I _really_ need to fix
get_maintainer.pl to _not_ pull in linux-raid and shli for DM-specific
changes!)