Messages in this thread Patch in this message | | | Date | Thu, 24 Feb 2022 10:27:54 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS |
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On Thu, Feb 24, 2022 at 04:24:38PM +0800, Junru Shen wrote: > Put the acquisition of the expected value inside the loop to prevent > an infinite loop when it does not match.
I suspect you've found this by inspection, as I don't beleive this can happen. See below.
> Signed-off-by: Junru Shen <hhusjrsjr@gmail.com> > --- > arch/x86/include/asm/atomic64_64.h | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/include/asm/atomic64_64.h b/arch/x86/include/asm/atomic64_64.h > index 7886d0578..3df04c44c 100644 > --- a/arch/x86/include/asm/atomic64_64.h > +++ b/arch/x86/include/asm/atomic64_64.h > @@ -207,9 +207,10 @@ static inline void arch_atomic64_and(s64 i, atomic64_t *v) > > static inline s64 arch_atomic64_fetch_and(s64 i, atomic64_t *v) > { > - s64 val = arch_atomic64_read(v); > + s64 val; > > do { > + val = arch_atomic64_read(v); > } while (!arch_atomic64_try_cmpxchg(v, &val, val & i)); ^^^^ See this bit above? ----------------------------'
If arch_atomic64_try_cmpxchg() fails, it writes the value in memory back to this address, so it has already done the equivalent of arch_atomic64_read(v).
If you're seing this go wrong, it implies that arch_atomic64_try_cmpxchg() is being mis-compiled, so please provide an example and the disassembly.
Likewise for the other instances below.
Thanks, Mark.
> return val; > } > @@ -225,9 +226,10 @@ static inline void arch_atomic64_or(s64 i, atomic64_t *v) > > static inline s64 arch_atomic64_fetch_or(s64 i, atomic64_t *v) > { > - s64 val = arch_atomic64_read(v); > + s64 val; > > do { > + val = arch_atomic64_read(v); > } while (!arch_atomic64_try_cmpxchg(v, &val, val | i)); > return val; > } > @@ -243,9 +245,10 @@ static inline void arch_atomic64_xor(s64 i, atomic64_t *v) > > static inline s64 arch_atomic64_fetch_xor(s64 i, atomic64_t *v) > { > - s64 val = arch_atomic64_read(v); > + s64 val; > > do { > + val = arch_atomic64_read(v); > } while (!arch_atomic64_try_cmpxchg(v, &val, val ^ i)); > return val; > } > -- > 2.30.2 >
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