* Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 3) sched_yield() makes me shudder
>
> CPU0 CPU1
>
> taskA
> lock(x->lock)
>
> preemption
> taskC
> taskB
> lock(y->lock);
> x = y->x;
> if (!try_lock(x->lock)) {
> unlock(y->lock);
> boost(taskA);
> sched_yield(); <- returns immediately
So I'm still struggling with properly parsing the usecase.
If y->x might become invalid the moment we drop y->lock, what makes the 'taskA'
use (after we've dropped y->lock) safe? Shouldn't we at least also have a
task_get(taskA)/task_put(taskA) reference count, to make sure the boosted task
stays around?
And if we are into getting reference counts, why not solve it at a higher level
and get a reference count to 'x' to make sure it's safe to use? Then we could do:
lock(y->lock);
retry:
x = y->x;
if (!trylock(x->lock)) {
get_ref(x->count)
unlock(y->lock);
lock(x->lock);
lock(y->lock);
put_ref(x->count);
if (y->x != x) { /* Retry if 'x' got dropped meanwhile */
unlock(x->lock);
goto retry;
}
}
Or so.
Note how much safer this sequence is, and still just as fast in the common case
(which I suppose is the main motivation within dcache.c?).
Thanks,
Ingo
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