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SubjectRe: [PATCH v2 1/1] blk-mq: fix hang caused by freeze/unfreeze sequence
On Wed, Aug 10, 2016 at 10:42 AM, Roman Penyaev
<roman.penyaev@profitbricks.com> wrote:
> Hi,
>
> On Wed, Aug 10, 2016 at 5:55 AM, Tejun Heo <tj@kernel.org> wrote:
>> Hello,
>>
>> On Mon, Aug 08, 2016 at 01:39:08PM +0200, Roman Pen wrote:
>>> Long time ago there was a similar fix proposed by Akinobu Mita[1],
>>> but it seems that time everyone decided to fix this subtle race in
>>> percpu-refcount and Tejun Heo[2] did an attempt (as I can see that
>>> patchset was not applied).
>>
>> So, I probably forgot about it while waiting for confirmation of fix.
>> Can you please verify that the patchset fixes the issue? I can apply
>> the patchset right away.
>
> I have not checked your patchset but according to my understanding
> it should not fix *this* issue.

So, your patchset does not help (but for sure it helps for keeping
internal percpu-refcount members consistent, but that is not related
to this issue). That's the backtrace which I observe:

Call Trace:
[<ffffffff810ba8df>] ? vprintk_default+0x1f/0x30
[<ffffffff816a47f5>] schedule+0x35/0x80
[<ffffffff81336154>] blk_mq_freeze_queue_wait+0x124/0x1a0
[<ffffffff810a3f70>] ? wake_atomic_t_function+0x60/0x60
[<ffffffff8133821a>] blk_mq_freeze_queue+0x1a/0x20
[<ffffffff8133822e>] blk_freeze_queue+0xe/0x10
[<ffffffff81329cc2>] blk_cleanup_queue+0xe2/0x280

To ease reproduction I do the following:

-------------------------------------------
static int thread_fn(void *data)
{
struct blk_mq_tag_set *tags = data;
struct request_queue *q;

while (!kthread_should_stop()) {
q = blk_mq_init_queue(tags);
BUG_ON(q == NULL);
/*
* That is done by blk_register_queue(), but here
* we are reproducing blk-mq bug and do not require
* gendisk and friends. Just silently switch to percpu.
*/
percpu_ref_switch_to_percpu(&q->q_usage_counter);

msleep(prandom_u32_max(10));
blk_cleanup_queue(q);
}

return 0;
}
-------------------------------------------

o Start 2 threads (exactly 2, we need 2 queues for 1 shared tags)
o Pass same shared tags pointer for each thread
o Wait
o PROFIT

To make immediate reproduction this hunk can be applied:

@@ -129,6 +142,7 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
WARN_ON_ONCE(freeze_depth < 0);
if (!freeze_depth) {
+ msleep(1000);
percpu_ref_reinit(&q->q_usage_counter);
wake_up_all(&q->mq_freeze_wq);
}
--
Roman

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