Messages in this thread | | | Date | Tue, 9 Jun 2020 15:38:37 +0200 | From | Peter Zijlstra <> | Subject | Re: blk-softirq vs smp_call_function_single_async() |
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On Mon, Jun 08, 2020 at 09:42:54AM -0700, Christoph Hellwig wrote: > On Mon, Jun 08, 2020 at 06:40:09PM +0200, Peter Zijlstra wrote: > > 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.
OK, after having looked at this more, I think my initial analysis is actually wrong and this code should work as-is.
The thing that I missed yesterday is that we only add the request to the blk_cpu_done list in the IPI, this means that the race I described earlier is not in fact possible.
The IPI must happen for progress to be made.
And the same is true for blk_mq_force_complete_rq(), which also uses this csd.
> The irq_work API looks reasonable. What are the tradeoffs for > smp_call_single_async vs irq_work?
To still answer your question; irq_work_queue*() has an atomic op extra that allows for more convenient semantics -- but is in your case strictly superfluous.
Still, Jens' point about irq_work being smaller stands, and I think more users could benefit from something intermediate. Let me continue with the cleanups / audit and see what comes out at the end.
Thanks!
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