Kernelspace C11 atomics for MIPS

[John Baldwin]

On Tuesday, June 04, 2013 9:12:36 am mdf at freebsd.org wrote:
> <digression>Personally, I find both the C11 atomics and FreeBSD's annoying,
> since "acquire" and "release" semantics are basically an x86 ism.  PPC has
> no notion of this; it has sync and isync and lwsync instructions which are
> separate from the atomic set, but can be combined to create the same
> effect.  Except the PPC manual is exceptionally explicit about what
> guarantees sync provides; it gives a mathematical ordering on loads/stores
> i, j and which effects can be seen when.  "Acquire" and "Release" seem to
> be named because you kinda need one to acquire a lock and kinda need one to
> release it.  But the effect of ordering loads or stores or both doesn't
> need to be dependent on the store/load, so putting the two together is just
> an x86 convenience (and an annoyance on at least PPC).</digression>

Actually, it came from ia64 (at least for FreeBSD's), not x86. :)  However,
it is still useful to think about, and they are barriers with respect to the
load/store of the lock cookie.  The requirement that the "acquire" blocks any 
subsequent loads/stores in program order from occurring until after the 
operation on the lock cookie succeeds and that "release" prevents any 
loads/stores frmo moving past the operation on the lock cookie is not quite 
the same as a traditional read or write barrier.  acquire and release only 
require a barrier in one direction and enforce ordering on both reads and 
writes.

-- 
John Baldwin