Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT_COUNT

From: Linus Torvalds
Date: Mon Apr 08 2013 - 10:50:29 EST

On Mon, Apr 8, 2013 at 7:31 AM, Steven Rostedt <srostedt@xxxxxxxxxx> wrote:
> It requires gcc reordering the code to where a preempt can happen inside
> preempt_disable. And also put in a position where the preempt_disable
> code it gets added matters.
> Then if gcc does this, we need a page fault to occur with a get_user()
> operation, which in practice seldom happens as most get user operations
> are done on freshly modified memory.
> And then, it would require the page fault to cause a schedule. This is
> the most likely of the things needed to occur, but itself is not a
> problem.
> Then, the schedule would have to cause the data that is being protect by
> the preempt_disable() to be corrupted. Either by scheduling in another
> process that monkeys with the data. Or if it protects per-cpu data,
> scheduling to another CPU (for the SMP case only).
> If any of the above does not occur, then you wont see a bug.

Right. The gcc reordering is also hard to actually notice if it does
happen, so even testing the fix for this looks nontrivial.

Something like the appended (whitespace-damaged and TOTALLY UNTESTED)
might be sufficient, but..



diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 5a710b9c578e..465df1c13386 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -93,14 +93,17 @@ do { \


-#define preempt_disable() do { } while (0)
-#define sched_preempt_enable_no_resched() do { } while (0)
-#define preempt_enable_no_resched() do { } while (0)
-#define preempt_enable() do { } while (0)
-#define preempt_disable_notrace() do { } while (0)
-#define preempt_enable_no_resched_notrace() do { } while (0)
-#define preempt_enable_notrace() do { } while (0)
+/* This is only a barrier to other asms. Notably get_user/put_user */
+#define asm_barrier() asm volatile("")
+#define preempt_disable() asm_barrier()
+#define sched_preempt_enable_no_resched() asm_barrier()
+#define preempt_enable_no_resched() asm_barrier()
+#define preempt_enable() asm_barrier()
+#define preempt_disable_notrace() asm_barrier()
+#define preempt_enable_no_resched_notrace() asm_barrier()
+#define preempt_enable_notrace() asm_barrier()

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