Messages in this thread | | | Date | Tue, 18 Aug 2009 16:52:20 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] spinlock: __raw_spin_is_locked() should return true for UP |
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On Tue, 18 Aug 2009, Steven Rostedt wrote: > > > The thing is, some people may assert that a lock is held, but others could > > easily be looping until it's not held using something like > > > > while (spin_is_locked(lock)) > > cpu_relax(); > > Wouldn't something like that be really racey? And anyone doing such a > thing had better have that code within an #ifdef CONFIG_SMP.
Sure, it's hopefully inside a #ifdef CONFIG_SMP.
And no, it's not necessarily racy. Sure, it's race in itself if that's all you are doing, but I could imagine writing that kind of code if I knew some lock was likely held, and I wanted to avoid doing a "try_lock()" until it got released.
The point is, "spin_is_locked()" is simply not a well-defined operation in this case. It could go either way.
And for the original case, we actually have a function for that:
assert_spin_locked(x)
which goes away on UP. Exactly because
BUG_ON(!spin_is_locked(x))
is not a good thing to do!
> > so it's hard to tell whether it should return true or false in the case > > where spin-locking simply doesn't exist. > > Actually, I did have a case where I would use it and would expect a return > of 0. That was in the experimental printk code to see if it was safe to > wakeup the klogd. I once had a check of the current cpu runqueue lock is > locked, and if it was, not to wake up klogd. I'm sure there's other cases > like this as well.
Yeah, "spin_is_locked()" can be useful for those kinds of things. A heuristic for whether we should do something based on whether some other CPU holds it (or we migth have recursion).
Exactly like it can be useful for doing the BUG_ON thing. But in both cases it's a bit iffy.
> Thinking about it, UP probably should have spin_is_locked always return > false, but if you want to make sure you are not in a critical section > with the lock not held, then use assert_spin_locked, which on UP should be > a nop.
That's what we do. That said, I also think we should generally try to avoid the kind of code that depends on spin_is_locked always returning false, for the same reason we should try to avoid any code that depends on it always returning true.
Linus
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