Re: [PATCH] zbd: Restore check_swd()

[Bart Van Assche <bvanassche@acm.org>]

On Fri, 2018-09-28 at 08:54 -0600, Jens Axboe wrote:
> +	for (z = zb; z < ze; z++) {
> +		pthread_mutex_lock(&z->mutex);
> +		swd += z->wp - z->start;
> +	}
> +	pthread_mutex_lock(&f->zbd_info->mutex);
> 
> Can 'z' ever end up being f->zbd_info? That would surely explain the
> deadlock.

Hi Jens,

f->zbd_info and z have a different type so unless something very weird is going
on &z->mutex can't be identical to &f->zbd_info->mutex.

What I think is going on is traditional lock inversion: check_swd() triggers
lock inversion if it is called while any of the z->mutex objects are held. One
such mutex is held where Damien tried to restore the calls of this function. I
will submit a patch that inserts these calls elsewhere.

Bart.