On 01/14/2016 02:55 PM, Paul E. McKenney wrote:

OK, so it looks like Will was asking not about WRC+addr+addr, but instead
about WRC+sync+addr.

(He actually asked twice about this and that too but skip this)

I am guessing that the manual's "Older instructions which must be globally
performed when the SYNC instruction completes" provides the equivalent
of ARM/Power A-cumulativity, which can be thought of as transitivity
backwards in time.  This leads me to believe that your smp_mb() needs
to use SYNC rather than SYNC_MB, as was the subject of earlier spirited
discussion in this thread.

Don't be fooled here by words "ordered" and "completed" - it is HW 
design items and actually written poorly.
Just assume that SYNC_MB is absolutely the same as SYNC for any CPU and 
coherent device (besides performance). The difference can be in 
non-coherent devices because SYNC actually tries to make a barrier for 
them too. In some SoCs it is just the same because there is no need to 
barrier a non-coherent device (device register access usually strictly 
ordered... if there is no bridge in between).

Suppose you have something like this:
...
Does your hardware guarantee that it is not possible for all of r0,
r1, r2, and r3 to be equal to zero at the end of the test, assuming
that a, b, c, and d are all initially zero, and the four functions
above run concurrently?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

Another (more academic) case is this one, with x and y initially zero:

...
Does SYNC_MB() prohibit r1 == 1 && r2 == 0 && r3 == 1 && r4 == 0?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

Note: I am not sure about ANY past MIPS R2 CPU because that stuff is 
implemented some time but nobody made it in Linux kernel (it was used by 
some vendor for non-Linux system). For that reason my patch for 
lightweight SYNCs has an option - implement it or implement a generic 
SYNC. It is possible that some vendor did it in different way but nobody 
knows or test it. But as a minimum - SYNC must be implemented in 
spinlocks/atomics/bitops, in recent P5600 it is proven that read can 
pass write in atomics.

MIPS R6 is a different story, I verified lightweight SYNCs from the 
beginning and it also should use SYNCs.

- Leonid.