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authorGreg Thelen <gthelen@google.com>2013-10-30 13:56:20 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-10-30 14:27:03 -0700
commitbd09d9a35111b6ffc0c7585d3853d0ec7f9f1eb4 (patch)
treed2183dd9349217860e07bb12ba9a6b6a3820b7ba /arch
parent3017f079efd6af199b0852b5c425364513db460e (diff)
downloadlinux-bd09d9a35111b6ffc0c7585d3853d0ec7f9f1eb4.tar.gz
percpu: fix this_cpu_sub() subtrahend casting for unsigneds
this_cpu_sub() is implemented as negation and addition. This patch casts the adjustment to the counter type before negation to sign extend the adjustment. This helps in cases where the counter type is wider than an unsigned adjustment. An alternative to this patch is to declare such operations unsupported, but it seemed useful to avoid surprises. This patch specifically helps the following example: unsigned int delta = 1 preempt_disable() this_cpu_write(long_counter, 0) this_cpu_sub(long_counter, delta) preempt_enable() Before this change long_counter on a 64 bit machine ends with value 0xffffffff, rather than 0xffffffffffffffff. This is because this_cpu_sub(pcp, delta) boils down to this_cpu_add(pcp, -delta), which is basically: long_counter = 0 + 0xffffffff Also apply the same cast to: __this_cpu_sub() __this_cpu_sub_return() this_cpu_sub_return() All percpu_test.ko passes, especially the following cases which previously failed: l -= ui_one; __this_cpu_sub(long_counter, ui_one); CHECK(l, long_counter, -1); l -= ui_one; this_cpu_sub(long_counter, ui_one); CHECK(l, long_counter, -1); CHECK(l, long_counter, 0xffffffffffffffff); ul -= ui_one; __this_cpu_sub(ulong_counter, ui_one); CHECK(ul, ulong_counter, -1); CHECK(ul, ulong_counter, 0xffffffffffffffff); ul = this_cpu_sub_return(ulong_counter, ui_one); CHECK(ul, ulong_counter, 2); ul = __this_cpu_sub_return(ulong_counter, ui_one); CHECK(ul, ulong_counter, 1); Signed-off-by: Greg Thelen <gthelen@google.com> Acked-by: Tejun Heo <tj@kernel.org> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/include/asm/percpu.h3
1 files changed, 2 insertions, 1 deletions
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 0da5200ee79..b3e18f80030 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -128,7 +128,8 @@ do { \
do { \
typedef typeof(var) pao_T__; \
const int pao_ID__ = (__builtin_constant_p(val) && \
- ((val) == 1 || (val) == -1)) ? (val) : 0; \
+ ((val) == 1 || (val) == -1)) ? \
+ (int)(val) : 0; \
if (0) { \
pao_T__ pao_tmp__; \
pao_tmp__ = (val); \