linux-kernel - Re: [PATCH v2 04/10] block: revert back to synchronous request

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Message-ID: <20200420215147.GP11244@42.do-not-panic.com>
Date:   Mon, 20 Apr 2020 21:51:47 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2 04/10] block: revert back to synchronous request_queue
 removal

On Mon, Apr 20, 2020 at 02:11:13PM -0700, Bart Van Assche wrote:
> On 4/20/20 11:59 AM, Luis Chamberlain wrote:
> > On Sun, Apr 19, 2020 at 03:23:31PM -0700, Bart Van Assche wrote:
> > > On 4/19/20 12:45 PM, Luis Chamberlain wrote:
> > > > + * Decrements the refcount to the request_queue kobject, when this reaches
> > > > + * 0 we'll have blk_release_queue() called. You should avoid calling
> > > > + * this function in atomic context but if you really have to ensure you
> > > > + * first refcount the block device with bdgrab() / bdput() so that the
> > > > + * last decrement happens in blk_cleanup_queue().
> > > > + */
> > > 
> > > Is calling bdgrab() and bdput() an option from a context in which it is not
> > > guaranteed that the block device is open?
> > 
> > If the block device is not open, nope. For that blk_get_queue() can
> > be used, and is used by the block layer. This begs the question:
> > 
> > Do we have *drivers* which requires access to the request_queue from
> > atomic context when the block device is not open?
> 
> Instead of trying to answer that question, how about changing the references
> to bdgrab() and bdput() into references to blk_get_queue() and
> blk_put_queue()? I think if that change is made that we won't have to
> research what the answer to the bdgrab()/bdput() question is.

Yeah that's fine, now at least we'd have documented what should be avoided.

  Luis