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

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Date:	Mon, 24 Aug 2015 14:22:59 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: [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 <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Prarit Bhargava <prarit@...hat.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

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