On 6/8/20 10:40 AM, Peter Zijlstra wrote: > On Mon, Jun 08, 2020 at 09:33:42AM -0700, Christoph Hellwig wrote: >> 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? > > There isn't one, it was meant to be used with static allocations. > > Frederic proposed changing all these to irq_work, and I think I'll go do > that. First dinner though. That sounds good to me, and will also shrink struct request a bit, which is always nice. -- Jens Axboe
