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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 12, 2019 at 03:24:09PM +0000, Bernard Metzler wrote:
> -----"Jason Gunthorpe" <jgg@ziepe.ca> wrote: -----

> Hmmm, I don't yet get why we should test and clear atomically, if we
> clear anyway - is it because we want to avoid clearing a re-arm which
> happens just after testing and before clearing?
> (1) If the test was positive, we will call the CQ event handler,
> and per RDMA verbs spec, the application MUST re-arm the CQ after it
> got a CQ event, to get another one. So clearing it sometimes before
> calling the handler is right.
> (2) If the test was negative, a test and reset would not change
> anything.
> 
> Another complication -- test_and_set_bit() operates on a single
> bit, but we have to test two bits, and reset both, if one is
> set. Can we do that atomically, if we test the bits conditionally?
> I didn't find anything appropriate.

There's cmpxchg() loops that can do that.

	unsigned int new, val = atomic_read(&var);
	do {
		if (!(val & MY_TWO_BITS))
			break;

		new = val | MY_TWO_BITS;
	} while (!atomic_try_cmpxchg(&var, &val, new));

only problem is you probably shouldn't share atomic_t with userspace,
unless you put conditions on what archs you support.

> >And then I think all the weird barriers go away
> >
> >> >> @@ -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?
> >> 
> >> Another core (such as a concurrent RX operation) shall see this
> >> CQ being re-armed asap.
> >
> >'ASAP' is not a '2nd piece of data'. 
> >
> >AFAICT this requirement is just a normal atomic set_bit which does
> >also expedite making the change visible?
> 
> Absolutely!!
> good point....this is just a single flag we are operating on.
> And the weird barrier goes away ;)

atomic ops don't expedite anything, and memory barriers don't make
things happen asap.

That is; the stores from atomic ops can stay in store buffers just like
any other store, and memory barriers don't flush store buffers, they
only impose order between memops.