Messages in this thread | | | Date | Wed, 21 Aug 2013 21:04:14 -0400 | From | Waiman Long <> | Subject | Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation |
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On 08/21/2013 11:51 AM, Alexander Fyodorov wrote: > 21.08.2013, 07:01, "Waiman Long"<waiman.long@hp.com>: >> On 08/20/2013 11:31 AM, Alexander Fyodorov wrote: >>> Isn't a race possible if another thread acquires the spinlock in the window >>> between setting lock->locked to 0 and issuing smp_wmb()? Writes from >>> the critical section from our thread might be delayed behind the write to >>> lock->locked if the corresponding cache bank is busy. >> The purpose of smp_wmb() is to make sure that content in the cache will >> be flushed out to the memory in case the cache coherency protocol cannot >> guarantee a single view of memory content on all processor. > Linux kernel does not support architectures without cache coherency, and while using memory barriers just for flushing write buffers ASAP on cache-coherent processors might benefit performance on some architectures it will hurt performance on others. So it must not be done in architecture-independent code. > >> In other >> word, smp_wmb() is used to make other processors see that the lock has >> been freed ASAP. If another processor see that before smp_wmb(), it will >> be even better as the latency is reduced. As the lock holder is the only >> one that can release the lock, there is no race condition here. > No, I was talking about the window between freeing lock and issuing smp_wmb(). What I meant is: > 1) A = 0 > 2) CPU0 locks the spinlock protecting A. > 3) CPU0 writes 1 to A, but the write gets stalled because the corresponding cache bank is busy. > 4) CPU0 calls spin_unlock() and sets lock->locked to 0. > > If CPU1 does a spin_lock() right now, it will succeed (since lock->locked == 0). But the write to A hasn't reached cache yet, so CPU1 will see A == 0. > > More examples on this are in Documentation/memory-barriers.txt
In this case, we should have smp_wmb() before freeing the lock. The question is whether we need to do a full mb() instead. The x86 ticket spinlock unlock code is just a regular add instruction except for some exotic processors. So it is a compiler barrier but not really a memory fence. However, we may need to do a full memory fence for some other processors.
>> That is a legitimate question. I don't think it is a problem on x86 as >> the x86 spinlock code doesn't do a full mb() in the lock and unlock >> paths. > It does because "lock" prefix implies a full memory barrier. > >> The smp_mb() will be conditionalized depending on the ARCH_QSPINLOCK >> setting. The smp_wmb() may not be needed, but a compiler barrier should >> still be there. > Do you mean because of inline? That shouldn't be a problem because smp_mb() prohibits compiler from doing any optimizations across the barrier (thanks to the "volatile" keyword).
What I mean is that I don't want any delay in issuing the unlock instruction because of compiler rearrangement. So there should be at least a barrier() call at the end of the unlock function.
At this point, I am inclined to have either a smp_wmb() or smp_mb() at the beginning of the unlock function and a barrier() at the end.
> More on this in Documentation/memory-barriers.txt > >>> Also I don't understand why there are so many uses of ACCESS_ONCE() >>> macro. It does not guarantee memory ordering with regard to other CPUs, >>> so probably most of the uses can be removed (with exception of >>> lock_is_contended(), where it prohibits gcc from optimizing the loop away). >> All the lock/unlock code can be inlined and we don't know what the >> compiler will do to optimize code. The ACCESS_ONCE() macro is used to >> make sure that the compiler won't optimize away the actual fetch or >> write of the memory. Even if the compiler won't optimize away the memory >> access, adding the ACCESS_ONCE() macro won't have any extra overhead. So >> a more liberal use of it won't hurt performance. > If compiler optimized memory access away it would be a bug. And I'm not so sure about overhead... For example, on some VLIW architectures ACCESS_ONCE() might prohibit compiler from mixing other instructions to the unlock.
As the lock/unlock functions can be inlined, it is possible that a memory variable can be accessed earlier in the calling function and the stale copy may be used in the inlined lock/unlock function instead of fetching a new copy. That is why I prefer a more liberal use of ACCESS_ONCE() for safety purpose.
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