On Thu, Aug 11, 2016 at 11:26:30PM +0000, Mathieu Desnoyers wrote: > ----- On Jul 24, 2016, at 2:01 PM, Dave Watson [email protected] 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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