From d6f029130fb83b36fb709a187275b0494035d689 Mon Sep 17 00:00:00 2001 From: Nicolas Pitre Date: Wed, 21 Dec 2005 12:26:25 -0500 Subject: [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 Acked-by: Nick Piggin Signed-off-by: Linus Torvalds --- include/linux/preempt.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include') diff --git a/include/linux/preempt.h b/include/linux/preempt.h index d9a2f5254a51..5769d14d1e6a 100644 --- 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) -- cgit v1.2.3