Messages in this thread | | | Date | Thu, 21 Aug 2008 14:15:08 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [RFC PATCH] Writer-biased low-latency rwlock v8 |
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On Thu, 21 Aug 2008, Linus Torvalds wrote: > > That leaves 30 bits for readers. If you still think you need to "limit the > number of readers", then you aren't getting it.
Side note: the actual main rwlock thing is designed for a 64-bit word and the waiters separately as two 32-bit words, so it doesn't really do what I describe, but that's actually because the whole sleeping thing is _harder_ than a spinning thing, and has races with wakeups etc.
A spinning thing, in contrast, is pretty trivial.
So here's what I think your code should be like:
rdlock: movl $4,%eax lock ; xaddl %eax,(%rdi) testl $3,%eax jne __rdlock_slowpath ret
rwlock: xorl %eax,%eax movl $1,%edx lock ; cmpxchgl %edx,(%rdi) jne __rwlock_slowpath ret
rdunlock: lock ; subl $4,(%rdi) ret
rwunlock: lock ; andl $~1,(%rdi) ret
and I'm pretty damn sure that that should be totally sufficient for a spinning rwlock. The non-spinning one is more complex just because the unlock paths need to guarantee that something gets woken up, that just isn't an issue when you do spinlocks.
Now, in the slow-path: - on the rwlock slowpath side, set bit#1 to make sure that readers get caught in the slowpath - then do a *separate* count of how many pending readers and writers (ie the ones that got caught into the slowpath) you have (one word each is probably fine), and then the slowpaths can just do the right thing depending on whether there are pending readers/writers.
See? The main lock needs not worry about number of writers AT ALL, because it's totally irrelevant. So don't worry about running out of bits. You won't. Just put those counts somewhere else! The only thing that matters for the main lock word is whether there are active readers (30 bits), and whether there is an active writer (there can only ever be one: 1 bit), and whether new readers should be trapped (1 bit).
If you worry about overflows, you're doing something wrong.
Linus
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