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

On Fri, Jan 19 2018 at 11:41am -0500,
Jens Axboe [off-list ref] wrote:

On 1/19/18 9:37 AM, Ming Lei wrote:

quoted

On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote:

quoted

On 1/19/18 9:26 AM, Ming Lei wrote:

quoted

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)?