quoted

The compiler is within its rights to read a 32-bit quantity 16 bits at
at time, even on a 32-bit machine.  I would be glad to help pummel any
compiler writer that pulls such a dirty trick, but the C standard 
really
does permit this.

Yes, but we don't write code for these compilers.  There are countless 
pieces of kernel code which would break in this condition, and there 
doesn't seem to be any interest in fixing this.

"Other things are broken too".  Great argument :-)

Sequence points enforce read-after-write ordering, not 
write-after-write.

Sequence points order *all* side effects; sequence points exist in the
domain of the abstract sequential model of the C language only.  The
compiler translates that to machine code that is equivalent to that C
code under the "as-if" rule; but this is still in that abstract model,
which doesn't include things such as SMP, visibility by I/O devices,
store queues, etc. etc.

We flush writes with reads for MMIO because of this effect as well as 
the CPU/bus effects.

You cannot flush all MMIO writes with reads; this is a PCI-specific
thing.  And even then, you need more than just the read itself: you
have to make sure the read completed and returned data.

quoted

In short, please retain atomic_set()'s volatility, especially on those
architectures that declared the atomic_t's counter to be volatile.

Like i386 and x86_64?  These used to have volatile in the atomic_t 
declaration.  We removed it, and the sky did not fall.

And this proves what?  Lots of stuff "works" by accident.


Segher