On 9/23/19 7:06 PM, Tejun Heo wrote:
> finish_writeback_work() reads @done->waitq after decrementing
> @done->cnt. However, once @done->cnt reaches zero, @done may be freed
> (from stack) at any moment and @done->waitq can contain something
> unrelated by the time finish_writeback_work() tries to read it. This
> led to the following crash.
>
> "BUG: kernel NULL pointer dereference, address: 0000000000000002"
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> PGD 0 P4D 0
> Oops: 0002 [#1] SMP DEBUG_PAGEALLOC
> CPU: 40 PID: 555153 Comm: kworker/u98:50 Kdump: loaded Not tainted
> ...
> Workqueue: writeback wb_workfn (flush-btrfs-1)
> RIP: 0010:_raw_spin_lock_irqsave+0x10/0x30
> Code: 48 89 d8 5b c3 e8 50 db 6b ff eb f4 0f 1f 40 00 66 2e 0f 1f 84 00 00
> 00 00 00 0f 1f 44 00 00 53 9c 5b fa 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 05
> 48 89 d8 5b c3 89 c6 e8 fe ca 6b ff eb f2 66 90
> RSP: 0018:ffffc90049b27d98 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
> RDX: 0000000000000001 RSI: 0000000000000003 RDI: 0000000000000002
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
> R10: ffff889fff407600 R11: ffff88ba9395d740 R12: 000000000000e300
> R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
> FS: 0000000000000000(0000) GS:ffff88bfdfa00000(0000)
> knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000002 CR3: 0000000002409005 CR4: 00000000001606e0
> Call Trace:
> __wake_up_common_lock+0x63/0xc0
> wb_workfn+0xd2/0x3e0
> process_one_work+0x1f5/0x3f0
> worker_thread+0x2d/0x3d0
> kthread+0x111/0x130
> ret_from_fork+0x1f/0x30
>
> Fix it by reading and caching @done->waitq before decrementing
> @done->cnt.
That's some nice debugging work.
Reviewed-by: Jens Axboe <ax...@kernel.dk>
> Signed-off-by: Tejun Heo <t...@kernel.org>
> Debugged-by: Chris Mason <c...@fb.com>
> Fixes: 30638b0125e1 ("writeback: Generalize and expose wb_completion")
This seems wrong, though:
commit 5b9cce4c7eb0696558dfd4946074ae1fb9d8f05d
Author: Tejun Heo <t...@kernel.org>
Date: Mon Aug 26 09:06:52 2019 -0700
writeback: Generalize and expose wb_completion
--
Jens Axboe
