Messages in this thread | | | Date | Fri, 21 Aug 2015 07:50:05 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match |
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On 08/21/2015 02:51 AM, Ingo Molnar wrote: > > * Prarit Bhargava <prarit@redhat.com> 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. 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 <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 | 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 */ > > Ok, I think this is a good fix to improve the robustness of this primitive, unless > someone objects. > > I tried to find the CPU hotplug code that broke with cpu_online() returning -1 but > failed - all current mainline usage sites seem to be testing for nonzero in one > way or another. Could you please point it out?
I'm sorry Ingo, I think my description may have confused you. I was debugging a cpu hotplug issue[1] and did
printk("cpu %d cpu online status %d\n", cpu, cpu_online(cpu));
as a debug printk. This printed out
cpu 3 cpu online status -1
which was really confusing. That lead me down the rabbit hole of looking at the sbb assembler instruction in variable_test_bit() to figure out why I was seeing -1.
P.
[1] The bug looks like it has to do with the system's firmware, not cpu hotplug.
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