Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Jan 07, 2019 at 02:13:29PM -0500, Michael S. Tsirkin wrote:
> On Mon, Jan 07, 2019 at 11:02:36AM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 07, 2019 at 08:36:36AM -0500, Michael S. Tsirkin wrote:
> > > On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote:
> > > > On Sun, Jan 06, 2019 at 11:23:07PM -0500, Michael S. Tsirkin wrote:
> > > > > On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote:
> > > > > > On 2019/1/3 上午4:57, Michael S. Tsirkin wrote:
> > > > 
> > > > > > > +#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \
> > > > > > > +	!defined(ARCH_NEEDS_READ_BARRIER_DEPENDS)
> > > > > > > +
> > > > > > > +#define dependent_ptr_mb(ptr, val) ({					\
> > > > > > > +	long dependent_ptr_mb_val = (long)(val);			\
> > > > > > > +	long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val;	\
> > > > > > > +									\
> > > > > > > +	BUILD_BUG_ON(sizeof(val) > sizeof(long));			\
> > > > > > > +	OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val);			\
> > > > > > > +	(typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val);	\
> > > > > > > +})
> > > > > > > +
> > > > > > > +#else
> > > > > > > +
> > > > > > > +#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); })
> > > > > > 
> > > > > > 
> > > > > > So for the example of patch 4, we'd better fall back to rmb() or need a
> > > > > > dependent_ptr_rmb()?
> > > > > > 
> > > > > > Thanks
> > > > > 
> > > > > You mean for strongly ordered architectures like Intel?
> > > > > Yes, maybe it makes sense to have dependent_ptr_smp_rmb,
> > > > > dependent_ptr_dma_rmb and dependent_ptr_virt_rmb.
> > > > > 
> > > > > mb variant is unused right now so I'll remove it.
> > > > 
> > > > How about naming the thing: dependent_ptr() ? That is without any (r)mb
> > > > implications at all. The address dependency is strictly weaker than an
> > > > rmb in that it will only order the two loads in qestion and not, like
> > > > rmb, any prior to any later load.
> > > 
> > > So I'm fine with this as it's enough for virtio, but I would like to point out two things:
> > > 
> > > 1. E.g. on x86 both SMP and DMA variants can be NOPs but
> > > the madatory one can't, so assuming we do not want
> > > it to be stronger than rmp then either we want
> > > smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr()
> > > or we just will specify that dependent_ptr() works for
> > > both DMA and SMP.
> > > 
> > > 2. Down the road, someone might want to order a store after a load.
> > > Address dependency does that for us too. Assuming we make
> > > dependent_ptr a NOP on x86, we will want an mb variant
> > > which isn't a NOP on x86. Will we want to rename
> > > dependent_ptr to dependent_ptr_rmb at that point?
> > 
> > But x86 preserves store-after-load orderings anyway, and even Alpha
> > respects ordering from loads to dependent stores.  So what am I missing
> > here?
> > 
> > 							Thanx, Paul
> 
> Oh you are right. Stores are not reordered with older loads on x86.
> 
> So point 2 is moot. Sorry about the noise.
> 
> I guess at this point the only sticking point is the ECC compiler.
> I'm inclined to stick an mb() there, seeing as it doesn't even
> have spectre protection enabled. Slow but safe.

Well, there is a mention of DMA above, which on some systems throws in
a wild card.  I would certainly hope that DMA would integrate nicely
with the cache-coherence protocols these days, unlike 25 years ago,
but who knows?

							Thanx, Paul