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

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Date:   Wed, 23 Aug 2017 11:59:09 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Bart Van Assche <Bart.VanAssche@....com>, peterz@...radead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
        kernel-team@....com
Subject: Re: possible circular locking dependency detected [was: linux-next:
 Tree for Aug 22]

On Wed, Aug 23, 2017 at 11:36:49AM +0900, Sergey Senozhatsky wrote:
> 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

Hi Sergey,

Right. It should be enhanced.

> 
> > > >  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