Re: [PATCH] fix race with preempt_enable()

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Nicolas Pitre wrote:
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.


Nice catch, this is not ARM specific either of course.

kernel/sched.c:preempt_schedule() has an equivalent barrier after
subtracting the preempt count and before checking TIF_NEED_RESCHED,
so I think this is the correct fix.

Linus will you apply?

Signed-off-by: Nicolas Pitre <[email protected]>

---

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index d9a2f52..5769d14 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)

--
SUSE Labs, Novell Inc.

Send instant messages to your online friends http://au.messenger.yahoo.com -
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux