On Fri, Jan 11, 2019 at 05:27:30PM -0500, Chuck Lever wrote: > > > > On Jan 11, 2019, at 5:10 PM, Bruce Fields <bfields@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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 --b.