Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson@fb.com wrote:

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+static inline __attribute__((always_inline))
+bool rseq_finish(struct rseq_lock *rlock,
+ intptr_t *p, intptr_t to_write,
+ struct rseq_state start_value)

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This ABI looks like it will work fine for our use case. I don't think it
has been mentioned yet, but we may still need multiple asm blocks
for differing numbers of writes. For example, an array-based freelist push:

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void push(void *obj) {
if (index < maxlen) {
freelist[index++] = obj;
}
}

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would be more efficiently implemented with a two-write rseq_finish:

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rseq_finish2(&freelist[index], obj, // first write
&index, index + 1, // second write
...);

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Would pairing one rseq_start with two rseq_finish do the trick
there ?

Yes, two rseq_finish works, as long as the extra rseq management overhead
is not substantial.

I've added a commit implementing rseq_finish2() in my rseq volatile
dev branch. You can fetch it at:

https://github.com/compudj/linux-percpu-dev/tree/rseq-fallback

I also have a separate test and benchmark tree in addition to the
kernel selftests here:

https://github.com/compudj/rseq-test

I named the first write a "speculative" write, and the second write
the "final" write.

Would you like to extend the test cases to cover your intended use-case ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com