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

From: Chuck Lever <chuck.lever@oracle.com>
To: Bruce Fields <bfields@fieldses.org>
Cc: Trond Myklebust <trondmy@hammerspace.com>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()
Date: Fri, 25 Jan 2019 13:32:58 -0800	[thread overview]
Message-ID: <40A18BC9-E3F9-45D0-98FF-7BCA171A3E62@oracle.com> (raw)
In-Reply-To: <20190125203001.GA5972@fieldses.org>

> 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