Messages in this thread | | | Date | Wed, 07 Oct 2015 19:27:17 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match |
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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 <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: "H. Peter Anvin" <hpa@zytor.com> > Cc: x86@kernel.org > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Prarit Bhargava <prarit@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)); > >
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