On Tue, Feb 07, 2017 at 06:33:46PM +0100, Paolo Valente wrote:
> Hi,
> this patch is meant to show that, if the body of the hook exit_icq is executed
> from inside that hook, and not as deferred work, then a circular deadlock
> occurs.
>
> It happens if, on a CPU
> - the body of icq_exit takes the scheduler lock,
> - it does so from inside the exit_icq hook, which is invoked with the queue
> lock held
>
> while, on another CPU
> - bfq_bio_merge, after taking the scheduler lock, invokes bfq_bic_lookup,
> which, in its turn, takes the queue lock. bfq_bic_lookup needs to take such a
> lock, because it invokes ioc_lookup_icq.
>
> For more details, here is a lockdep report, right before the deadlock did occur.
>
> [ 44.059877] ======================================================
> [ 44.124922] [ INFO: possible circular locking dependency detected ]
> [ 44.125795] 4.10.0-rc5-bfq-mq+ #38 Not tainted
> [ 44.126414] -------------------------------------------------------
> [ 44.127291] sync/2043 is trying to acquire lock:
> [ 44.128918] (&(&bfqd->lock)->rlock){-.-...}, at: [<ffffffff90484195>] bfq_exit_icq_bfqq+0x55/0x140
> [ 44.134052]
> [ 44.134052] but task is already holding lock:
> [ 44.134868] (&(&q->__queue_lock)->rlock){-.....}, at: [<ffffffff9044738e>] put_io_context_active+0x6e/0xc0
Hey, Paolo,
I only briefly skimmed the code, but what are you using the queue_lock
for? You should just use your scheduler lock everywhere. blk-mq doesn't
use the queue lock, so the scheduler is the only thing you need mutual
exclusion against. I'm guessing if you stopped using that, your locking
issues would go away.
