Right. ROTFL... volatile actually breaks atomic_t instead of making
it safe. x++ becomes a register load, increment and a register store.
Without volatile we can increment the memory directly. It seems that
volatile requires that the variable is loaded into a register first
and then operated upon. Understandable when you think about volatile
being used to access memory mapped I/O registers where a RMW
operation could be problematic.
So, if we want consistent behavior, we're pretty much screwed unless
we use inline assembler everywhere?
Nah, this whole argument is flawed -- "without volatile" we still
*cannot* "increment the memory directly". On x86, you need a lock
prefix; on other archs, some other mechanism to make the memory
increment an *atomic* memory increment.
And no, RMW on MMIO isn't "problematic" at all, either.
An RMW op is a read op, a modify op, and a write op, all rolled
into one opcode. But three actual operations.
The advantages of asm code for atomic_{read,set} are:
1) all the other atomic ops are implemented that way already;
2) you have full control over the asm insns selected, in particular,
you can guarantee you *do* get an atomic op;
3) you don't need to use "volatile <data>" which generates
not-all-that-good code on archs like x86, and we want to get rid
of it anyway since it is problematic in many ways;
4) you don't need to use *(volatile <type>*)&<data>, which a) doesn't
exist in C; b) isn't documented or supported in GCC; c) has a recent
history of bugginess; d) _still uses volatile objects_; e) _still_
is problematic in almost all those same ways as in 3);
5) you can mix atomic and non-atomic accesses to the atomic_t, which
you cannot with the other alternatives.
The only disadvantage I know of is potentially slightly worse
instruction scheduling. This is a generic asm() problem: GCC
cannot see what actual insns are inside the asm() block.
Segher
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