* [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS
@ 2022-02-24 8:24 Junru Shen
2022-02-24 10:13 ` Boqun Feng
` (2 more replies)
0 siblings, 3 replies; 5+ messages in thread
From: Junru Shen @ 2022-02-24 8:24 UTC (permalink / raw)
To: Will Deacon
Cc: Peter Zijlstra, Boqun Feng, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, Dave Hansen, x86, H . Peter Anvin, linux-kernel,
Junru Shen
Put the acquisition of the expected value inside the loop to prevent an infinite loop when it does not match.
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));
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
^ permalink raw reply related [flat|nested] 5+ messages in thread* Re: [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS
2022-02-24 8:24 [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS Junru Shen
@ 2022-02-24 10:13 ` Boqun Feng
2022-02-24 10:27 ` Mark Rutland
2022-02-24 12:02 ` Peter Zijlstra
2 siblings, 0 replies; 5+ messages in thread
From: Boqun Feng @ 2022-02-24 10:13 UTC (permalink / raw)
To: Junru Shen
Cc: Will Deacon, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, Dave Hansen, x86, H . Peter Anvin, linux-kernel
Hi,
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.
>
Is there a small test showing a infinite loop can really happen? If so,
have you tested it?
Note that arch_atomic64_try_cmpxchg() will refresh 'val' if the previous
value doesn't match the content of 'v'.
Regards,
Boqun
> 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));
> 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
>
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS
2022-02-24 8:24 [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS Junru Shen
2022-02-24 10:13 ` Boqun Feng
@ 2022-02-24 10:27 ` Mark Rutland
2022-02-24 12:02 ` Peter Zijlstra
2 siblings, 0 replies; 5+ messages in thread
From: Mark Rutland @ 2022-02-24 10:27 UTC (permalink / raw)
To: Junru Shen
Cc: Will Deacon, Peter Zijlstra, Boqun Feng, Thomas Gleixner,
Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H . Peter Anvin,
linux-kernel
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
>
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS
2022-02-24 8:24 [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS Junru Shen
2022-02-24 10:13 ` Boqun Feng
2022-02-24 10:27 ` Mark Rutland
@ 2022-02-24 12:02 ` Peter Zijlstra
2 siblings, 0 replies; 5+ messages in thread
From: Peter Zijlstra @ 2022-02-24 12:02 UTC (permalink / raw)
To: Junru Shen
Cc: Will Deacon, Boqun Feng, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, Dave Hansen, x86, H . Peter Anvin, linux-kernel
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.
NAK, this is broken.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS
@ 2022-02-24 8:24 Junru Shen
0 siblings, 0 replies; 5+ messages in thread
From: Junru Shen @ 2022-02-24 8:24 UTC (permalink / raw)
To: Will Deacon
Cc: Peter Zijlstra, Boqun Feng, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, Dave Hansen, x86, H . Peter Anvin, linux-kernel,
Junru Shen
Put the acquisition of the expected value inside the loop to prevent an infinite loop when it does not match.
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));
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
^ permalink raw reply related [flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-02-24 12:03 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2022-02-24 8:24 [PATCH] atomic: Put the fetching of the old value into the loop when doing atomic CAS Junru Shen
2022-02-24 10:13 ` Boqun Feng
2022-02-24 10:27 ` Mark Rutland
2022-02-24 12:02 ` Peter Zijlstra
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2022-02-24 8:24 Junru Shen
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