linux-kernel - Re: [PATCH -next v2] blk-mq: fix panic during blk_mq_run_work

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Date:   Fri, 20 May 2022 15:02:13 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH -next v2] blk-mq: fix panic during blk_mq_run_work_fn()

在 2022/05/20 14:23, yukuai (C) 写道:
> 在 2022/05/20 11:44, Ming Lei 写道:
>> On Fri, May 20, 2022 at 11:25:42AM +0800, Yu Kuai wrote:
>>> Our test report a following crash:
>>>
>>> BUG: kernel NULL pointer dereference, address: 0000000000000018
>>> PGD 0 P4D 0
>>> Oops: 0000 [#1] SMP NOPTI
>>> CPU: 6 PID: 265 Comm: kworker/6:1H Kdump: loaded Tainted: G           
>>> O      5.10.0-60.17.0.h43.eulerosv2r11.x86_64 #1
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
>>> rel-1.12.1-0-ga5cab58-20220320_160524-szxrtosci10000 04/01/2014
>>> Workqueue: kblockd blk_mq_run_work_fn
>>> RIP: 0010:blk_mq_delay_run_hw_queues+0xb6/0xe0
>>> RSP: 0018:ffffacc6803d3d88 EFLAGS: 00010246
>>> RAX: 0000000000000006 RBX: ffff99e2c3d25008 RCX: 00000000ffffffff
>>> RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffff99e2c911ae18
>>> RBP: ffffacc6803d3dd8 R08: 0000000000000000 R09: ffff99e2c0901f6c
>>> R10: 0000000000000018 R11: 0000000000000018 R12: ffff99e2c911ae18
>>> R13: 0000000000000000 R14: 0000000000000003 R15: ffff99e2c911ae18
>>> FS:  0000000000000000(0000) GS:ffff99e6bbf00000(0000) 
>>> knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 0000000000000018 CR3: 000000007460a006 CR4: 00000000003706e0
>>> Call Trace:
>>>   __blk_mq_do_dispatch_sched+0x2a7/0x2c0
>>>   ? newidle_balance+0x23e/0x2f0
>>>   __blk_mq_sched_dispatch_requests+0x13f/0x190
>>>   blk_mq_sched_dispatch_requests+0x30/0x60
>>>   __blk_mq_run_hw_queue+0x47/0xd0
>>>   process_one_work+0x1b0/0x350
>>>   worker_thread+0x49/0x300
>>>   ? rescuer_thread+0x3a0/0x3a0
>>>   kthread+0xfe/0x140
>>>   ? kthread_park+0x90/0x90
>>>   ret_from_fork+0x22/0x30
>>>
>>> After digging from vmcore, I found that the queue is cleaned
>>> up(blk_cleanup_queue() is done) and tag set is
>>> freed(blk_mq_free_tag_set() is done).
>>>
>>> There are two problems here:
>>>
>>> 1) blk_mq_delay_run_hw_queues() will only be called from
>>> __blk_mq_do_dispatch_sched() if e->type->ops.has_work() return true.
>>> This seems impossible because blk_cleanup_queue() is done, and there
>>> should be no io. Commit ddc25c86b466 ("block, bfq: make bfq_has_work()
>>> more accurate") fix the problem in bfq. And currently ohter schedulers
>>> don't have such problem.
>>>
>>> 2) 'hctx->run_work' still exists after blk_cleanup_queue().
>>> blk_mq_cancel_work_sync() is called from blk_cleanup_queue() to cancel
>>> all the 'run_work'. However, there is no guarantee that new 'run_work'
>>> won't be queued after that(and before blk_mq_exit_queue() is done).
>>
>> It is blk_mq_run_hw_queue() caller's responsibility to grab
>> ->q_usage_counter for avoiding queue cleaned up, so please fix the user
>> side.
>>
> Hi,
> 
> Thanks for your advice.
> 
> blk_mq_run_hw_queue() can be called async, in order to do that, what I
> can think of is that grab 'q_usage_counte' before queuing 'run->work'
> and release it after. Which is very similar to this patch...

Hi,

How do you think about following change:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index cedc355218db..7d5370b5b5e1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1627,8 +1627,16 @@ static void __blk_mq_delay_run_hw_queue(struct 
blk_mq_hw_ctx *hctx, bool async,
                 put_cpu();
         }

+       /*
+        * No need to queue work if there is no io, and this can avoid race
+        * with blk_cleanup_queue().
+        */
+       if (!percpu_ref_tryget(&hctx->queue->q_usage_counter))
+               return;
+
         kblockd_mod_delayed_work_on(blk_mq_hctx_next_cpu(hctx), 
&hctx->run_work,
                                     msecs_to_jiffies(msecs));
+       percpu_ref_put(&hctx->queue->q_usage_counter);
  }