Wrong fix, though. Instead we should change it to use the set instruction, which would also make it easier to use the CC_SET/CC_OUT proposed macros to use assembly out in the future.
The downside with set is that it only sets a single byte, the upside is that it always outputs 0 or 1, and apparently if the output variable is your bool gcc can use that for optimization. On August 20, 2015 11:51:03 PM PDT, Ingo Molnar <mi...@kernel.org> wrote: > >* Prarit Bhargava <pra...@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 <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 */ > >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? > >Thanks, > > Ingo -- Sent from my Android device with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
