fall-back-on-interrupt-disable-in-cmpxchg8b-on-80386-and-80486

fall-back-on-interrupt-disable-in-cmpxchg8b-on-80386-and-80486

Actually, on 386, cmpxchg and cmpxchg_local fall back on
cmpxchg_386_u8/16/32: it disables interruptions around non atomic
updates to mimic the cmpxchg behavior.

The comment:
/* Poor man's cmpxchg for 386. Unsuitable for SMP */

already present in cmpxchg_386_u32 tells much about how this cmpxchg
implementation should not be used in a SMP context. However, the cmpxchg_local
can perfectly use this fallback, since it only needs to be atomic wrt the local
cpu.

This patch adds a cmpxchg_486_u64 and uses it as a fallback for cmpxchg64
and cmpxchg64_local on 80386 and 80486.

Q:
but why is it called cmpxchg_486 when the other functions are called

A:
Because the standard cmpxchg is missing only on 386, but cmpxchg8b is
missing both on 386 and 486.

Citing Intel's Instruction set reference:

cmpxchg:
This instruction is not supported on Intel processors earlier than the
Intel486 processors.

cmpxchg8b:
This instruction encoding is not supported on Intel processors earlier
than the Pentium processors.

Q:
What's the reason to have cmpxchg64_local on 32 bit architectures?
Without that need all this would just be a few simple defines.

A:
cmpxchg64_local on 32 bits architectures takes unsigned long long
parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b
to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense
to provide a flavor of cmpxchg and cmpxchg_local using this instruction.

Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it
makes sense _not_ to define cmpxchg64 while cmpxchg could still be
available.

Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a

However, cmpxchg64_local will be emulated by disabling interrupts on all
architectures where it is not supported atomically.

Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it
would make the 386/486 fallbacks ugly, make its design different from
cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot
be emulated) and require the __cmpxchg_local to be expressed as a macro
rather than an inline function so the parameters would not be fixed to
unsigned long long in every case.

So I think cmpxchg64_local makes sense there, but I am open to
suggestions.

Q:
Are there any callers?

A:
I am actually using it in LTTng in my timestamping code. I use it to
work around CPUs with asynchronous TSCs. I need to update 64 bits
values atomically on this 32 bits architecture.

Changelog:
- Ran though checkpatch.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>