Re: [PATCH] Fix cpu iterator on empty bitmask

[Rusty Russell]

On Wed, 2004-04-28 at 10:05, William Lee Irwin III wrote:
> On Wed, Apr 28, 2004 at 09:52:53AM +1000, Rusty Russell wrote:
> > Name: Fix cpumask iterator over empty cpu set
> > Status: Trivial
> > Can't use _ffs() without first checking for zero, and if bits beyond
> > NR_CPUS set it'll give bogus results.  Use find_first_bit
> 
> I sent in something equivalent to this along with a number of other
> fixes (cpus_shift_right() leaking junk bits in in and cpus_weight()
> and cpus_empty() and cpus_equal() and the like not ignoring tails) and
> got a NAK since it clashes with something Paul Jackson's doing.

Agreed, I'm pretty sure Paul's work doesn't make this mistake, but this
is a trivial patch for a real big which is causing oopses today.

Linus, please apply...
Rusty.

Name: Fix cpumask iterator over empty cpu set
Status: Trivial

Can't use _ffs() without first checking for zero, and if bits beyond
NR_CPUS set it'll give bogus results.  Use find_first_bit

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .26180-linux-2.6.6-rc2-bk5/include/asm-generic/cpumask_arith.h .26180-linux-2.6.6-rc2-bk5.updated/include/asm-generic/cpumask_arith.h
--- .26180-linux-2.6.6-rc2-bk5/include/asm-generic/cpumask_arith.h	2004-01-10 13:59:33.000000000 +1100
+++ .26180-linux-2.6.6-rc2-bk5.updated/include/asm-generic/cpumask_arith.h	2004-04-28 09:50:23.000000000 +1000
@@ -43,7 +43,7 @@
 #define cpus_promote(map)		({ map; })
 #define cpumask_of_cpu(cpu)		({ ((cpumask_t)1) << (cpu); })
 
-#define first_cpu(map)			__ffs(map)
+#define first_cpu(map)			find_first_bit(&(map), NR_CPUS)
 #define next_cpu(cpu, map)		find_next_bit(&(map), NR_CPUS, cpu + 1)
 
 #endif /* __ASM_GENERIC_CPUMASK_ARITH_H */

-- 
Anyone who quotes me in their signature is an idiot -- Rusty Russell

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