Messages in this thread |  | | Date | Wed, 6 Dec 2006 12:41:53 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it |
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On Wed, Dec 06, 2006 at 11:34:52AM -0800, Linus Torvalds wrote: > On Wed, 6 Dec 2006, Matthew Wilcox wrote: > > Given parisc's paucity of atomic operations (load-and-zero-32bit and > > load-and-zero-64bit), cmpxchg() is impossible to implement safely. > > There has to be something we can hook to exclude another processor > > modifying the variable. I'm OK with using atomic_cmpxchg(); we have > > atomic_set locked against it. > > How do you to the atomic bitops?
The same way we do atomic_t.
What I hadn't realised (because I hadn't read dhowell's implementation ... because it hasn't shown up on git2.kernel.org yet) is that he doesn't actually *use* this unlocked-assignment that would cause the problem. He uses bitops which use the same locks.
> Also, I don't see quite why you think "cmpxchg()" and "atomic_cmpxchg()" > would be different. ANY cmpxchg() needs to be atomic - if it's not, > there's no point to the operation at all, since you'd just write it as > > if (*p == x) > *p = y; > > instead, and not use cmpxchg().
The difference is that we can (and do) acquire a lock for atomic_set. We can't acquire one for X = 6.
> - on SMP, use a spinlock (with interrupts disabled), and share that > spinlock with all the other atomic ops (bitops at a minimum - the > "atomic_t" operations have traditionally been in another "locking > space" because of sparc32 historic braindamage, but I'd suggest > sharing the same spinlock between them all).
Yep, we agree.
> And yeah, it sucks. You _can_ (if you really want to) make the spinlock be > hashed based on the address of the atomic data structure. That at least > allows you to do _multiple_ spinlocks, but let's face it, your real > problem is _likely_ going to be cacheline bouncing, not contention, and > then using a hashed lock won't be likely to buy you all that much.
We do hash based on the address -- and we try to arrange things such that different spinlocks are acquired for different cachelines. I don't know if anyone's benchmarked it recently to see how well it performs. It's a bit of a waltzing bear [1] at times ;-)
[1] The wonder is not how well it waltzes, but that it waltzes at all - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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