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

[Ming Lei <ming.lei@redhat.com>]

On Fri, May 20, 2022 at 04:49:19PM +0800, yukuai (C) wrote:
> 在 2022/05/20 16:34, Ming Lei 写道:
> > On Fri, May 20, 2022 at 03:02:13PM +0800, yukuai (C) wrote:
> > > 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:
> > > 
> > 
> > I think the issue is in blk_mq_map_queue_type() which may touch tagset.
> > 
> > So please try the following patch:
> > 
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index ed1869a305c4..5789e971ac83 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2174,8 +2174,7 @@ static bool blk_mq_has_sqsched(struct request_queue *q)
> >    */
> >   static struct blk_mq_hw_ctx *blk_mq_get_sq_hctx(struct request_queue *q)
> >   {
> > -	struct blk_mq_hw_ctx *hctx;
> > -
> > +	struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
> >   	/*
> >   	 * If the IO scheduler does not respect hardware queues when
> >   	 * dispatching, we just don't bother with multiple HW queues and
> > @@ -2183,8 +2182,8 @@ static struct blk_mq_hw_ctx *blk_mq_get_sq_hctx(struct request_queue *q)
> >   	 * just causes lock contention inside the scheduler and pointless cache
> >   	 * bouncing.
> >   	 */
> > -	hctx = blk_mq_map_queue_type(q, HCTX_TYPE_DEFAULT,
> > -				     raw_smp_processor_id());
> > +	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, 0, ctx);
> > +
> >   	if (!blk_mq_hctx_stopped(hctx))
> >   		return hctx;
> >   	return NULL;
> 
> Hi, Ming
> 
> This patch do make sense, however, this doesn't fix the root cause, it

Isn't the root cause that tagset is referred after blk_cleanup_queue
returns?

> just bypass the problem like commit ddc25c86b466 ("block, bfq: make
> bfq_has_work() more accurate"), which will prevent
> blk_mq_delay_run_hw_queues() to be called in such case.

How can?

> 
> I do think we need to make sure 'run_work' doesn't exist after
> blk_cleanup_queue().

Both hctx and request queue are fine to be referred after blk_cleanup_queue
returns, what can't be referred is tagset.


Thanks,
Ming