On Mon, Jun 08, 2020 at 08:45:57AM -0700, Christoph Hellwig wrote:
> On Mon, Jun 08, 2020 at 01:58:00PM +0200, Peter Zijlstra wrote:
> > Hi Jens,
> >
> > I've been going through smp_call_function_single_async() users and
> > stumbled upon blk-softirq.c, which has:
> >
> > static int raise_blk_irq(int cpu, struct request *rq)
> > {
> > if (cpu_online(cpu)) {
> > call_single_data_t *data = &rq->csd;
> >
> > data->func = trigger_softirq;
> > data->info = rq;
> > data->flags = 0;
> >
> > smp_call_function_single_async(cpu, data);
> > return 0;
> > }
> >
> > return 1;
> > }
> >
> > What, if anything, guarantees rq->csd is not already in use at that
> > time?
>
> 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?
