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

[Bruce Fields <bfields@fieldses.org>]

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.

--b.