Re: [dm-devel] [PATCH V4] blk-mq: introduce BLK_STS_DEV_RESOURCE

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"Bart" == Bart Van Assche [off-list ref] writes:

Bart> On Mon, 2018-01-29 at 16:44 -0500, Mike Snitzer wrote:

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But regardless of which wins the race, the queue will have been run.
Which is all we care about right?

Bart> Running the queue is not sufficient. With this patch applied it can happen
Bart> that the block driver returns BLK_STS_DEV_RESOURCE, that the two or more
Bart> concurrent queue runs finish before sufficient device resources are available
Bart> to execute the request and that blk_mq_delay_run_hw_queue() does not get
Bart> called at all. If no other activity triggers a queue run, e.g. request
Bart> completion, this will result in a queue stall.

Doesn't this argue that you really want some sort of completions to be
run in this case instead?  Instead of busy looping or waiting for a
set amount of time, just fire off a callback to run once you have the
resources available, no?

I can't provide any code examples, I don't know the code base nearly
well enough.

John