include/linux/percpu_counter.h: race in uniprocessor percpu_counter_add()

The percpu interface is supposed to be preempt and irq safe.

But:
The uniprocessor implementation of percpu_counter_add() is not irq safe:
if an interrupt happens during the +=, then the result is undefined.

Therefore: switch from preempt_disable() to local_irq_save().
This prevents interrupts from interrupting the +=, and as a side effect
prevents preemption.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Manfred Spraul <[e-mail]>
Cc: "Sun, Jiebin" <[e-mail]>
Cc: <[e-mail]>
Cc: Alexander Sverdlin <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/percpu_counter.h

···
 static inline void
 percpu_counter_add(struct percpu_counter *fbc, s64 amount)
 {
-	preempt_disable();
+	unsigned long flags;
+
+	local_irq_save(flags);
 	fbc->count += amount;
-	preempt_enable();
+	local_irq_restore(flags);
 }
 
 /* non-SMP percpu_counter_add_local is the same with percpu_counter_add */