linux-kernel - Re: [RFC 3/3] block: avoid deferral of blk_release

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Date:   Thu, 9 Apr 2020 18:11:11 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Nicolai Stange <nstange@...e.de>
Cc:     Bart Van Assche <bvanassche@....org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, mhocko@...e.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [RFC 3/3] block: avoid deferral of blk_release_queue() work

On Thu, Apr 02, 2020 at 04:49:37PM +0200, Nicolai Stange wrote:
> Bart Van Assche <bvanassche@....org> writes:
> 
> > On 2020-04-01 17:00, Luis Chamberlain wrote:
> > The description of this patch mentions a single blk_release_queue() call
> > that happened in the past from a context from which sleeping is not
> > allowed and from which sleeping is allowed today. Have all other
> > blk_release_queue() / blk_put_queue() calls been verified to see whether
> > none of these happens from a context from which sleeping is not allowed?
> 
> I've just done this today and found the following potentially
> problematic call paths to blk_put_queue().
> 
> 1.) mem_cgroup_throttle_swaprate() takes a spinlock and
>     calls blkcg_schedule_throttle()->blk_put_queue().
> 
>     Also note that AFAICS mem_cgroup_try_charge_delay() can be called
>     with GFP_ATOMIC.

I have a solution to this which would avoid having to deal with the
concern completely. I'll post in my follow up.

> 2.) scsi_unblock_requests() gets called from a lot of drivers and
>     invoke blk_put_queue() through
>     scsi_unblock_requests() -> scsi_run_host_queues() ->
>     scsi_starved_list_run() -> blk_put_queue().

sd_probe() calls device_add_disk(), and the scsi lib also has its
own refcounting for scsi, but unless you call sd_remove() you'll be
protecting the underlying block disk and request_queue, as sd_remove()
calls the del_gendisk() which would in call call blk_unregister_queue()
which calls the last blk_put_queue(). If sd_remove() can be called from
atomic context we can also fix this, and this should be evident how in
my next follow up series of patches.

  Luis