Re: blk-mq crash under KVM in multiqueue block code (with virtio-blk and ext4)

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> >>> Does anyone have an idea?
> >>> The request itself is completely filled with cc
> >>
> >> That is very weird, the 'rq' is got from hctx->tags,  and rq should be
> >> valid, and rq->q shouldn't have been changed even though it was
> >> double free or double allocation.
> >>
> >>> I am currently asking myself if blk_mq_map_request should protect against softirq here but I cant say for sure,as I have never looked into that code before.
> >>
> >> No, it needn't the protection.
> >>
> >> Thanks,
> >>
> > 
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> 

Digging through the code, I think I found a possible cause:

tags->rqs[..] is not initialized with zeroes (via alloc_pages_node in
blk-mq.c:blk_mq_init_rq_map()).

When a request is created:

1. __blk_mq_alloc_request() gets a free tag (thus e.g. removing it from
bitmap_tags)

2. __blk_mq_alloc_request() initializes is via blk_mq_rq_ctx_init(). The struct
is filled with life and rq->q is set.


When blk_mq_hw_ctx_check_timeout() is called:

1. blk_mq_tag_busy_iter() is used to call blk_mq_timeout_check() on all busy
tags.

2. This is done by collecting all free tags using bt_for_each_free() and
handing them to blk_mq_timeout_check(). This uses bitmap_tags.

3. blk_mq_timeout_check() calls  blk_mq_tag_to_rq() to get the rq.


Could we have a race between

- getting the tag (turning it busy) and initializing it and
- detecting a tag to be busy and trying to access it?


I haven't looked at the details yet. If so, we might either do some locking
(if there is existing infrastructure), or somehow mark a request as not being
initialized prior to accessing the data.