lkml.org 
[lkml]   [2020]   [Jun]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: blk-softirq vs smp_call_function_single_async()
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?


\
 
 \ /
  Last update: 2020-06-08 17:59    [W:1.439 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site