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SubjectRe: mmotm 2009-04-10-02-21 uploaded - forkbombed by work_for_cpu

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 13 Apr 2009, Andrew Morton wrote:
> >
> > static void drv_write(struct drv_cmd *cmd)
> > {
> > - unsigned int i;
> > + unsigned int cpu;
> >
> > - for_each_cpu(i, cmd->mask) {
> > - work_on_cpu(i, do_drv_write, cmd);
> > - }
> > + for_each_cpu(cpu, cmd->mask)
> > + smp_call_function_single(cpu, do_drv_write, cmd, 1);
>
> Ok, that's just -wrong-.
>
> Doesn't anybody else see anything odd in doing
>
> for_each_cpu(cpu, cmd->mask)
> smp_call_function_single(cpu, ..);
>
> and react to it?
>
> IOW, why not just do
>
> smp_call_function_many(cmd->mask, do_drv_write, cmd, 1);
>
> here?

Uhm, yes. The ancient version did:

for_each_cpu_mask(i, cmd->mask) {
set_cpus_allowed(current, cpumask_of_cpu(i));
do_drv_write(cmd);
}
and we just kept that pattern while somewhat mindlessly converting
it around, and never realized how it collapses to a nice masked API
version when the right IPI-call primitive is used.

Ingo


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