Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

Sergey Senozhatsky
2017-08-23T02:36:49Z

On (08/23/17 09:03), Byungchul Park wrote:
[..]

aha, ok

> The report is talking about the following lockup:
>
> A work in a worker A task work on exit to user
> ------------------ ---------------------------
> mutex_lock(&bdev->bd_mutex)
> mutext_lock(&bdev->bd_mutex)
> blk_execute_rq()
> wait_for_completion_io_timeout(&A)
> complete(&A)
>
> Is this impossible?

I was really confused how this "unlock" may lead to a deadlock

> > > other info that might help us debug this:
> > > Possible unsafe locking scenario by crosslock:
> > > CPU0 CPU1
> > > ---- ----
> > > lock(&bdev->bd_mutex);
> > > lock((complete)&wait#2);
> > > lock(&bdev->bd_mutex);
> > > unlock((complete)&wait#2);

any chance the report can be improved? mention timeout, etc?
// well, if this functionality will stay.

p.s.
Bart Van Assche, thanks for Cc-ing Park Byungchul, I was really
sure I didn't enabled the cross-release, but apparently I was wrong:
CONFIG_LOCKDEP_CROSSRELEASE=y
CONFIG_LOCKDEP_COMPLETIONS=y

-ss