-----"Jason Gunthorpe" <jgg@xxxxxxxx> wrote: ----- >To: "Bernard Metzler" <BMT@xxxxxxxxxxxxxx> >From: "Jason Gunthorpe" <jgg@xxxxxxxx> >Date: 07/12/2019 02:03PM >Cc: "Arnd Bergmann" <arnd@xxxxxxxx>, "Doug Ledford" ><dledford@xxxxxxxxxx>, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, >linux-rdma@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx >Subject: [EXTERNAL] Re: [PATCH] rdma/siw: avoid smp_store_mb() on a >u64 > >On Fri, Jul 12, 2019 at 11:33:46AM +0000, Bernard Metzler wrote: >> >diff --git a/drivers/infiniband/sw/siw/siw_verbs.c >> >b/drivers/infiniband/sw/siw/siw_verbs.c >> >index 32dc79d0e898..41c5ab293fe1 100644 >> >+++ b/drivers/infiniband/sw/siw/siw_verbs.c >> >@@ -1142,10 +1142,11 @@ int siw_req_notify_cq(struct ib_cq >*base_cq, >> >enum ib_cq_notify_flags flags) >> > >> > if ((flags & IB_CQ_SOLICITED_MASK) == IB_CQ_SOLICITED) >> > /* CQ event for next solicited completion */ >> >- smp_store_mb(*cq->notify, SIW_NOTIFY_SOLICITED); >> >+ WRITE_ONCE(*cq->notify, SIW_NOTIFY_SOLICITED); >> > else >> > /* CQ event for any signalled completion */ >> >- smp_store_mb(*cq->notify, SIW_NOTIFY_ALL); >> >+ WRITE_ONCE(*cq->notify, SIW_NOTIFY_ALL); >> >+ smp_wmb(); >> > >> > if (flags & IB_CQ_REPORT_MISSED_EVENTS) >> > return cq->cq_put - cq->cq_get; >> >> >> Hi Arnd, >> Many thanks for pointing that out! Indeed, this CQ notification >> mechanism does not take 32 bit architectures into account. >> Since we have only three flags to hold here, it's probably better >> to make it a 32bit value. That would remove the issue w/o >> introducing extra smp_wmb(). > >I also prefer not to see smp_wmb() in drivers.. > >> I'd prefer smp_store_mb(), since on some architectures it shall be >> more efficient. That would also make it sufficient to use >> READ_ONCE. > >The READ_ONCE is confusing to me too, if you need store_release >semantics then the reader also needs to pair with load_acquite - >otherwise it doesn't work. > I used READ_ONCE since it is a potentially user land shared object. Since we are under spinlock, we might just do a matching WRITE_ONCE instead of the smp_store_mb() ?? Many thanks! Bernard. >Still, we need to do something rapidly to fix the i386 build, please >revise right away.. > >Jason > >
