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. This can be easily
resolved by adding a "!!" to force 0 or 1 as a return.
It looks like the code never does, for example, (test_bit() == 1) so this
change should not have any impact.
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 | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index cfe3b95..a87a5fb 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -320,7 +320,7 @@ static inline int variable_test_bit(long nr, volatile const
unsigned long *addr)
: "=r" (oldbit)
: "m" (*(unsigned long *)addr), "Ir" (nr));
- return oldbit;
+ return !!oldbit;
}
#if 0 /* Fool kernel-doc since it doesn't do macros yet */
--
1.7.9.3
--
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