Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()

Chuck Lever
2019-01-25T21:33:09Z

> On Jan 25, 2019, at 12:30 PM, Bruce Fields <bfields@fieldses.org> wrote:
>
> On Mon, Jan 14, 2019 at 12:24:24PM -0500, Chuck Lever wrote:
>>
>>
>>> On Jan 11, 2019, at 7:56 PM, Bruce Fields <bfields@fieldses.org> wrote:
>>>
>>> On Fri, Jan 11, 2019 at 05:27:30PM -0500, Chuck Lever wrote:
>>>>
>>>>
>>>>> On Jan 11, 2019, at 5:10 PM, Bruce Fields <bfields@fieldses.org> wrote:
>>>>>
>>>>> On Fri, Jan 11, 2019 at 04:54:01PM -0500, Chuck Lever wrote:
>>>>>>> On Jan 11, 2019, at 4:52 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>>>>>>>> So, I think we need your patch plus something like this.
>>>>>>>>
>>>>>>>> Chuck, maybe you could help me with the "XXX: Chuck:" parts?
>>>>>>>
>>>>>>> I haven't been following. Why do you think those are necessary?
>>>>>
>>>>> I'm worried something like this could happen:
>>>>>
>>>>> CPU 1 CPU 2
>>>>> ----- -----
>>>>>
>>>>> set XPT_DATA dec xpt_nr_rqsts
>>>>>
>>>>> svc_xprt_enqueue svc_xprt_enqueue
>>>>>
>>>>> And both decide nothing should be done if neither sees the change that
>>>>> the other made.
>>>>>
>>>>> Maybe I'm still missing some reason that couldn't happen.
>>>>>
>>>>> Even if it can happen, it's an unlikely race that will likely be fixed
>>>>> when another event comes along a little later, which would explain why
>>>>> we've never seen any reports.
>>>>>
>>>>>>> We've had set_bit and atomic_{inc,dec} in this code for ages,
>>>>>>> and I've never noticed a problem.
>>>>>>>
>>>>>>> Rather than adding another CPU pipeline bubble in the RDMA code,
>>>>>>> though, could you simply move the set_bit() call site inside the
>>>>>>> critical sections?
>>>>>>
>>>>>> er, inside the preceding critical section. Just reverse the order
>>>>>> of the spin_unlock and the set_bit.
>>>>>
>>>>> That'd do it, thanks!
>>>>
>>>> I can try that here and see if it results in a performance regression.
>>>
>>> Thanks, I've got a version with a typo fixed at
>>>
>>> git://linux-nfs.org/~bfields/linux.git nfsd-next
>>
>> Applied all four patches here. I don't see any performance regressions,
>> but my server has only a single last-level CPU cache.
>
> Thanks!
>
> I'm adding a Tested-by: for you if that's OK.

Sorry, yes! that's fine with me.

--
Chuck Lever