[PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match

From: Prarit Bhargava
Date: Mon Aug 24 2015 - 14:23:11 EST

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 <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
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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