[PATCH] fix race with preempt_enable()

Currently a simple

	void foo(void) { preempt_enable(); }

produces the following code on ARM:

foo:
	bic	r3, sp, #8128
	bic	r3, r3, #63
	ldr	r2, [r3, #4]
	ldr	r1, [r3, #0]
	sub	r2, r2, #1
	tst	r1, #4
	str	r2, [r3, #4]
	blne	preempt_schedule
	mov	pc, lr

The problem is that the TIF_NEED_RESCHED flag is loaded _before_ the
preemption count is stored back, hence any interrupt coming within that
3 instruction window causing TIF_NEED_RESCHED to be set won't be
seen and scheduling won't happen as it should.

Nothing currently prevents gcc from performing that reordering.  There
is already a barrier() before the decrement of the preemption count, but
another one is needed between this and the TIF_NEED_RESCHED flag test
for proper code ordering.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index d9a2f5254a51194858091bec44bf6bfb43cb9168..5769d14d1e6ab658b7ab8b2a2c9de117ec2b4838 100644 (file)
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -48,6 +48,7 @@ do { \
 #define preempt_enable() \
 do { \
        preempt_enable_no_resched(); \
+       barrier(); \
        preempt_check_resched(); \
 } while (0)