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

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Message-ID: <20150822091414.GA18050@gmail.com>
Date:	Sat, 22 Aug 2015 11:14:14 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on
 a match


* Prarit Bhargava <prarit@...hat.com> wrote:

> 
> 
> On 08/21/2015 02:51 AM, Ingo Molnar wrote:
> > 
> > * Prarit Bhargava <prarit@...hat.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@...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 |    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.

Ok, fair enough!

Still worth fixing IMHO.

Thanks,

	Ingo
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