Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

Linus Torvalds
2009-01-07T20:55:49Z

On Wed, 7 Jan 2009, Steven Rostedt wrote:
>
> Next comes the issue to know if the owner is still running. Wouldn't we
> need to do something like
>
> if (task_thread_info(cpu_rq(cpu)->curr) == owner)

Yes. After verifying that "cpu" is in a valid range.

> I understand that this should not be a problem, but I'm afraid it will
> give me nightmares at night. ;-)
>
> God that code had better be commented well.

Well, the good news is that it really would be just a few - admittedly
very subtle - lines, each basically generating just a couple of machine
instructions. So we'd be looking at code where the actual assembly output
should hopefully be in the ten-to-twenty instruction range, and the C code
itself would be about five times as many comments as actual real lines.

So the code really shouldn't be much worse than

/*
* Look out! "thread" is an entirely speculative pointer
* access and not reliable.
*/
void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread)
{
for (;;) {
unsigned cpu;
struct runqueue *rq;

if (lock->owner != thread)
break;

/*
* Need to access the cpu field knowing that
* DEBUG_PAGEALLOC could have unmapped it if
* the mutex owner just released it and exited.
*/
if (__get_user(cpu, &thread->cpu))
break;

/*
* Even if the access succeeded (likely case),
* the cpu field may no longer be valid. FIXME:
* this needs to validate that we can do a
* get_cpu() and that we have the percpu area.
*/
if (cpu >= NR_CPUS)
break;

if (!cpu_online(cpu))
break;

/*
* Is that thread really running on that cpu?
*/
rq = cpu_rq(cpu);
if (task_thread_info(rq->curr) != thread)
break;

cpu_relax();
}
}

and it all looks like it shouldn't be all that bad. Yeah, it's like 50
lines of C code, but it's mostly comments about subtle one-liners that
really expand to almost no real code at all.

Linus