Re: [PATCH -next v2] blk-mq: fix panic during blk_mq_run_work_fn()
From: yukuai (C)
Date: Fri May 20 2022 - 06:56:40 EST
在 2022/05/20 17:53, Ming Lei 写道:
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?
No, it's not the root cause. If we can make sure 'hctx->run_work' won't
exist after blk_cleanup_queue(), such problem won't be triggered.
Actually, blk_cleaup_queue() already call blk_mq_cancel_work_sync() to
do that, however, new 'hctx->run_work' can be queued after that.
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?
See the 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 -> hctx->run_work
details how blk_mq_delay_run_hw_queues() is called:
__blk_mq_do_dispatch_sched
if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
break -> has_work has to return true.
rq = e->type->ops.dispatch_request(hctx);
if (!rq)
run_queue = true
break; -> dispatch has to failed
if (run_queue)
blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
Thus if 'has_work' is accurate, blk_mq_delay_run_hw_queues() won't be
called if there is no io.
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.
I agree with that, however, I think we still need to reach an agreement
about root cause of this problem...
Thanks,
Kuai