Re: [PATCH v2 1/1] blk-mq: fix hang caused by freeze/unfreeze sequence

[Roman Penyaev <roman.penyaev@profitbricks.com>]

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