Messages in this thread | | | Date | Wed, 7 Jan 2009 12:55:49 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning |
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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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