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

[Linus Torvalds]

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
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