lib: percpu_counter_sub

Hugh spotted that some code does:
  percpu_counter_add(&counter, -unsignedlong)

which, when the amount argument is of type s32, sort-of works thanks to
two's-complement. However when we'd change the type to s64 this breaks on 32bit
machines, because the promotion rules zero extend the unsigned number.

Provide percpu_counter_sub() to hide the s64 cast. That is:
  percpu_counter_sub(&counter, foo)
is equal to:
  percpu_counter_add(&counter, -(s64)foo);

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index b84fc8667de8..438a170187ec 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -105,4 +105,9 @@ static inline void percpu_counter_dec(struct percpu_counter *fbc)
         percpu_counter_add(fbc, -1);
 }
 
+static inline void percpu_counter_sub(struct percpu_counter *fbc, s64 amount)
+{
+        percpu_counter_add(fbc, -amount);
+}
+
 #endif /* _LINUX_PERCPU_COUNTER_H */