linux-kernel - Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

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Message-ID: <20180119164722.GA29449@redhat.com>
Date:   Fri, 19 Jan 2018 11:47:22 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Ming Lei <ming.lei@...hat.com>,
        Bart Van Assche <Bart.VanAssche@....com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "osandov@...com" <osandov@...com>
Subject: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

On Fri, Jan 19 2018 at 11:41am -0500,
Jens Axboe <axboe@...nel.dk> wrote:

> On 1/19/18 9:37 AM, Ming Lei wrote:
> > On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote:
> >> On 1/19/18 9:26 AM, Ming Lei wrote:
> >>> On Fri, Jan 19, 2018 at 09:19:24AM -0700, Jens Axboe wrote:
> >>
> >> There are no pending requests for this case, nothing to restart the
> >> queue. When you fail that blk_get_request(), you are idle, nothing
> >> is pending.
> > 
> > I think we needn't worry about that, once a device is attached to
> > dm-rq, it can't be mounted any more, and usually user don't use the device
> > directly and by dm-mpath at the same time.
> 
> Even if it doesn't happen for a normal dm setup, it is a case that
> needs to be handled. The request allocation is just one example of
> a wider scope resource that can be unavailable. If the driver returns
> NO_DEV_RESOURCE (or whatever name), it will be a possibility that
> the device itself is currently idle.

How would a driver's resources be exhausted yet the device is idle (so
as not to be able to benefit from RESTART)?