Re: Re: [PATCH] rdma/siw: avoid smp_store_mb() on a u64

Jason Gunthorpe
2019-07-12T13:53:43Z

On Fri, Jul 12, 2019 at 01:05:14PM +0000, Bernard Metzler wrote:
>
> >To: "Bernard Metzler" <BMT@zurich.ibm.com>
> >From: "Jason Gunthorpe" <jgg@ziepe.ca>
> >Date: 07/12/2019 02:03PM
> >Cc: "Arnd Bergmann" <arnd@arndb.de>, "Doug Ledford"
> ><dledford@redhat.com>, "Peter Zijlstra" <peterz@infradead.org>,
> >linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org
> >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.
> >
> >Still, we need to do something rapidly to fix the i386 build, please
> >revise right away..
> >
> >Jason
> >
> >
>
> We share CQ (completion queue) notification flags between application
> (which may be user land) and producer (kernel QP's (queue pairs)).
> Those flags can be written by both application and QP's. The application
> writes those flags to let the driver know if it shall inform about new
> work completions. It can write those flags at any time.
> Only a kernel producer reads those flags to decide if
> the CQ notification handler shall be kicked, if a new CQ element gets
> added to the CQ. When kicking the completion handler, the driver resets the
> notification flag, which must get re-armed by the application.

This looks wrong to me.. a userspace notification re-arm cannot be
lost, so have a split READ/TEST/WRITE sequence can't possibly work?

I'd expect an atomic test and clear here?

> @@ -1141,11 +1145,17 @@ int siw_req_notify_cq(struct ib_cq *base_cq, enum ib_cq_notify_flags flags)
> siw_dbg_cq(cq, "flags: 0x%02x\n", flags);
>
> if ((flags & IB_CQ_SOLICITED_MASK) == IB_CQ_SOLICITED)
> - /* CQ event for next solicited completion */
> - smp_store_mb(*cq->notify, SIW_NOTIFY_SOLICITED);
> + /*
> + * Enable CQ event for next solicited completion.
> + * and make it visible to all associated producers.
> + */
> + smp_store_mb(cq->notify->flags, SIW_NOTIFY_SOLICITED);

But what is the 2nd piece of data to motivate the smp_store_mb?

Jason