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

[Boqun Feng]

On Thu, Aug 11, 2016 at 11:26:30PM +0000, Mathieu Desnoyers wrote:
> ----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson@xxxxxx wrote:
> 
> >>> +static inline __attribute__((always_inline))
> >>> +bool rseq_finish(struct rseq_lock *rlock,
> >>> + intptr_t *p, intptr_t to_write,
> >>> + struct rseq_state start_value)
> > 
> >>> 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:
> > 
> >>> void push(void *obj) {
> >>> if (index < maxlen) {
> >>> freelist[index++] = obj;
> >>> }
> >>> }
> > 
> >>> would be more efficiently implemented with a two-write rseq_finish:
> > 
> >>> rseq_finish2(&freelist[index], obj, // first write
> >>> &index, index + 1, // second write
> >>> ...);
> > 
> >> 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.
> 

Maybe I miss something subtle, but if the first write is only a
"speculative" write, why can't we put it in the rseq critical section
rather than asm block? Like this:

	do_rseq(..., result, targetptr, newval
		{	
			newval = index;
			targetptr = &index;
			if (newval < maxlen)
				freelist[newval++] = obj;
			else
				result = false;
		}

No extra rseq_finish() is needed here, but maybe a little more
"speculative" writes?

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

Dave, if you are going to write some test cases about your use-cases,
would you also try the away I mentioned above?


Besides, do we allow userspace programs do read-only access to the
memory objects modified by do_rseq(). If so, we have a problem when
there are two writes in a do_rseq()(either in the rseq critical section
or in the asm block), because in current implemetation, these two writes
are unordered, which makes the readers outside a do_rseq() could observe
the ordering of writes differently.

For rseq_finish2(), a simple solution would be making the "final" write
a RELEASE.

Regards,
Boqun

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

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