Re: [PATCH -next v2] blk-mq: fix panic during blk_mq_run_work_fn()

[yukuai (C)]

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);
 }