Re: softirq->blk_mq_make_request deadlocks

[Jens Axboe <axboe@kernel.dk>]

On 5/22/18 8:29 AM, Vitaly Mayatskih wrote:
> Hi,
> 
> I'm working on a new network block device and see occasional deadlocks
> when trying to submit_bio from softirq (network rcv handler). This may
> be a new use case for blk-mq, but I think context spinlock should be
> really taken with bh disabled. I *seem* can avoid the deadlock if bio
> has BIO_NOMERGE set, but I need to merge bios for better network
> utilization (no merge costs about 15% of bandwidth). Did I miss
> something, or the lock indeed needs no bh for that case (recursive
> ctx->lock in softirq)?

You can't call submit_bio() from irq/soft irq context, it will
potentially sleep for a new request. The various locks for blk-mq
have been carefully designed _not_ to need irq/bh disabling, but
that's really orthogonal to the previous comment which is your
main issue.

-- 
Jens Axboe