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SubjectRe: [PATCH 3/6] locking/atomic/x86: Use READ_ONCE before atomic{,64}_try_cmpxchg loops
On Tue, Apr 09, 2024 at 12:03:54PM +0200, Uros Bizjak wrote:
> The value preload before the cmpxchg loop does not need to be atomic,
> but should use READ_ONCE to prevent compiler from merging, refetching
> or reordering the read.
>

Yes, and that's what arch_atomic_read() and arch_atomic64_read() do...

> This patch unifies arch_atomic{,64}_{,fetch}_{and,or,xor}() macros
> between x86_32 and x86_64 targets.
>
> No functional changes intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> arch/x86/include/asm/atomic.h | 8 ++++----
> arch/x86/include/asm/atomic64_64.h | 20 ++++++++++----------
> 2 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> index 55a55ec04350..b166da21ee98 100644
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -20,7 +20,7 @@ static __always_inline int arch_atomic_read(const atomic_t *v)
> * Note for KASAN: we deliberately don't use READ_ONCE_NOCHECK() here,
> * it's non-inlined function that increases binary size and stack usage.
> */
> - return __READ_ONCE((v)->counter);
> + return __READ_ONCE(v->counter);

Removing the unncessary brackets is fine, but the commit message didn't mention this.

[...]

> static __always_inline int arch_atomic_fetch_and(int i, atomic_t *v)
> {
> - int val = arch_atomic_read(v);
> + int val = __READ_ONCE(v->counter);

This is the wrong thing to do; arch_atomic_read() already has the required
semantic, and it more clearly aligns with the use of arch_atomic_try_cmpxchg()
below. It contains the documentation regarding why we use __READ_ONCE()
specifically (which we should probably note in arch_atomic64_read()).

Please leave this as-is, and likewise for the other cases below. Similarly, the
prior patch should use arch_atomic{,_64}_read() rather than using
__READ_ONCE().

[...]

> static __always_inline s64 arch_atomic64_fetch_and(s64 i, atomic64_t *v)
> {
> - s64 val = arch_atomic64_read(v);
> + s64 val = __READ_ONCE(v->counter);
> +
> + do { } while (!arch_atomic64_try_cmpxchg(v, &val, val & i));
>
> - do {
> - } while (!arch_atomic64_try_cmpxchg(v, &val, val & i));
> return val;

I think this reformatting is what you meant in the commit message when you said:

| This patch unifies arch_atomic{,64}_{,fetch}_{and,or,xor}() macros
| between x86_32 and x86_64 targets.

Assuming so, can you please jsut do that, and say:

This patch reformats the x86_64 arch_atomic{,64}_{,fetch}_{and,or,xor}()
functions to match the x86_32 versions.

Mark.

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