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 <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Prarit Bhargava <pra...@redhat.com>
---
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
--
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/
