Messages in this thread | | | Date | Tue, 9 Apr 2024 12:07:05 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 3/6] locking/atomic/x86: Use READ_ONCE before atomic{,64}_try_cmpxchg loops |
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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.
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> 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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