----- On Aug 11, 2016, at 11:10 PM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:
> ----- On Aug 11, 2016, at 9:28 PM, Boqun Feng boqun.feng@xxxxxxxxx wrote:
>
>> 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?
>
> This won't work unfortunately. The speculative stores need to be
> between the rseq_event_counter comparison instruction in the rseq_finish
> asm sequence and the final store. The ip fixup is really needed for
> correctness of speculative stores. The sequence number scheme only works
> for loads.
>
> Putting it in the C code between rseq_start and rseq_finish would lead
> to races such as:
>
> thread A thread B
> rseq_start
> <preempted>
> <sched in>
> rseq_start
> freelist[offset + 1] = obj
> rseq_finish
> offset++
> <preempted>
> <sched in>
> freelist[newval + 1] = obj <--- corrupts the list content.
>
Small clarification to the scenario:
thread A thread B
rseq_start
load offset into (register 1)
<preempted>
<sched in>
rseq_start
freelist[offset + 1] = obj
rseq_finish
offset++
<preempted>
<sched in>
freelist[(register 1) + 1] = obj <--- corrupts the list content.
Thanks,
Mathieu
> <snip>
>
>> 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.
>
> Indeed, we would need a release semantic for the final store here if this
> is the common use. Or we could duplicate the "flavors" of rseq_finish2 and
> add a rseq_finish2_release. We should find a way to eliminate code duplication
> there. I suspect we'll end up doing macros.
>
> Thanks,
>
> Mathieu
>
>>
>> Regards,
>> Boqun
>>
>>> Thanks,
>>>
>>> Mathieu
>>>
>>> --
>>> Mathieu Desnoyers
>>> EfficiOS Inc.
>> > http://www.efficios.com
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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