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SubjectRe: blk-softirq vs smp_call_function_single_async()
On Mon, Jun 08, 2020 at 05:58:33PM +0200, Peter Zijlstra wrote:
> > A request can only be completed once.
>
> Sure, but that doesn't help.
>
> CPU0 CPU1
>
> raise_blk_irq() BLOCK_SOFTIRQ
> IPI -> CPU1
>
> // picks up thing from CPU0
> req->complete(req);
>
>
> <big hole where CSD is active and request completed>
>
> <IPI>
> trigger_softirq()
>
>
> What happens to a struct request after completion, is it free()d
> or reused? If reused, how do we guarantee CSD completion before
> free()ing?

The request is freed in the block layer sense by __blk_mq_free_request
(which doesn't actually free the memory, so it eventually gets reused).

__blk_mq_free_request is called from blk_mq_end_request which
is called from the drivers ->complete_rq method, which is called
from the block softirq.

What is the method to guaranteed CSD completion?

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