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

[Peter Zijlstra]

On Wed, Aug 23, 2017 at 09:03:04AM +0900, Byungchul Park wrote:
> On Tue, Aug 22, 2017 at 09:43:56PM +0000, Bart Van Assche wrote:

> 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?
> 
> To Peterz,
> 
> Anyway I wanted to avoid lockdep reports in the case using a timeout
> interface. Do you think it's still worth reporting the kind of lockup?

Yes, people might not have expected to hit the timeout on this. They
might think timeout means a dead device or something like that.

I'd like to heard from the block folks if this was constructed thus on
purpose though.