Messages in this thread | | | Date | Thu, 21 Jun 2007 13:58:15 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [patch] spinlock debug: make looping nicer |
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On Thu, 21 Jun 2007, Ingo Molnar wrote: > > btw., back then we also tried a spin_is_locked() based inner loop but it > didnt help the ->tree_lock lockups either. In any case i very much agree > that the 'nicer' looping should be added again - the patch below does > that. (build and boot tested)
Ok, I'm definitely not going to apply it right now, though.
> and the reason that this didnt help the ->tree_lock lockup is likely the > same why wait_task_inactive() broke _independently_ of the 'niceness' of > the spin-lock operation: there were too few instructions between > releasing the lock and re-acquiring it again can cause permanent > starvation of another CPU. No amount of logic on the spinning side can > overcome this, if acquire/release critical sections are following each > other too fast.
Exactly.
The only way to handle that case is to make sure that the person who *gets* the spinlock will slow down. The person who doesn't get it can't do anything at all about the fact that he's locked out.
A way to do that (as already mentioned) is to have a "this lock is contended" flag, and have the person who gets the lock do something about it (where the "something" might actually be as simple as saying "When I release a lock that somebody marked as having lots of contention, I will clear the contention flag, and then just delay myself").
Side note: that trivial approach only really helps for a *single* thread that gets it very much (like the example in wait_task_inactive). For true contention with multiple different CPU's that can *all* have the bad behaviour, you do actually need real queueing.
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