[PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match
This issue was noticed while debugging a CPU hotplug issue. On x86
with (NR_CPUS > 1) the cpu_online() define is cpumask_test_cpu().
cpumask_test_cpu() should return 1 if the cpu is set in cpumask and
0 otherwise.
However, cpumask_test_cpu() returns -1 if the cpu in the cpumask is
set and 0 otherwise. This happens because cpumask_test_cpu() calls
test_bit() which is a define that will call variable_test_bit().
variable_test_bit() calls the assembler instruction sbb (Subtract
with Borrow, " Subtracts the source from the destination, and subtracts 1
extra if the Carry Flag is set. Results are returned in "dest".)
A bit match results in -1 being returned from variable_test_bit() if a
match occurs, not 1 as the function is supposed to.
It looks like the code never does, for example, (test_bit() == 1) so this
change should not have any impact.
[v2]: hpa: Use setc, (Set if Carry, "Sets the byte in the operand to 1 if
the Carry Flag is set, otherwise sets the operand to 0.") instead of !!
Cc: Thomas Gleixner
Cc: Ingo Molnar
Cc: "H. Peter Anvin"
Cc: x...@kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Prarit Bhargava
---
arch/x86/include/asm/bitops.h |4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index cfe3b95..c0bff87 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -313,10 +313,10 @@ static __always_inline int constant_test_bit(long nr,
const volatile unsigned lo
static inline int variable_test_bit(long nr, volatile const unsigned long
*addr)
{
- int oldbit;
+ u8 oldbit;
asm volatile("bt %2,%1\n\t"
-"sbb %0,%0"
+"setc %0"
: "=r" (oldbit)
: "m" (*(unsigned long *)addr), "Ir" (nr));
--
1.7.9.3
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Re: [PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match
re-ping on this. Just making sure this wasn't dropped on the floor.
P.
On 08/24/2015 02:22 PM, Prarit Bhargava wrote:
> This issue was noticed while debugging a CPU hotplug issue. On x86
> with (NR_CPUS > 1) the cpu_online() define is cpumask_test_cpu().
> cpumask_test_cpu() should return 1 if the cpu is set in cpumask and
> 0 otherwise.
>
> However, cpumask_test_cpu() returns -1 if the cpu in the cpumask is
> set and 0 otherwise. This happens because cpumask_test_cpu() calls
> test_bit() which is a define that will call variable_test_bit().
>
> variable_test_bit() calls the assembler instruction sbb (Subtract
> with Borrow, " Subtracts the source from the destination, and subtracts 1
> extra if the Carry Flag is set. Results are returned in "dest".)
>
> A bit match results in -1 being returned from variable_test_bit() if a
> match occurs, not 1 as the function is supposed to.
>
> It looks like the code never does, for example, (test_bit() == 1) so this
> change should not have any impact.
>
> [v2]: hpa: Use setc, (Set if Carry, "Sets the byte in the operand to 1 if
> the Carry Flag is set, otherwise sets the operand to 0.") instead of !!
>
> Cc: Thomas Gleixner
> Cc: Ingo Molnar
> Cc: "H. Peter Anvin"
> Cc: x...@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Prarit Bhargava
> ---
> arch/x86/include/asm/bitops.h |4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index cfe3b95..c0bff87 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -313,10 +313,10 @@ static __always_inline int constant_test_bit(long nr,
> const volatile unsigned lo
>
> static inline int variable_test_bit(long nr, volatile const unsigned long
> *addr)
> {
> - int oldbit;
> + u8 oldbit;
>
> asm volatile("bt %2,%1\n\t"
> - "sbb %0,%0"
> + "setc %0"
>: "=r" (oldbit)
>: "m" (*(unsigned long *)addr), "Ir" (nr));
>
>
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Re: [PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match
* Prarit Bhargava wrote: > re-ping on this. Just making sure this wasn't dropped on the floor. So I didn't apply it back when I saw your patch because I didn't see where you addressed/analyzed the second paragraph of hpa's review: "The downside with set is that it only sets a single byte, the upside is that it always outputs 0 or 1, and apparently if the output variable is your bool gcc can use that for optimization." That's a valid concern and should be addressed. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
