Re: RCU: non-atomic assignment to long/pointer variables in gcc

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 15, 2013 at 02:30:32PM +0400, Konstantin Khlebnikov wrote:
> Documentation/atomic_ops.txt (182dd4b277177e8465ad11cd9f85f282946b5578)
> says that pointers, longs, ints, and chars are stored and loaded atomically.
> 
> But GCC actually may split assignment to 'long' variable into two instructions.
> see example in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981
> 
> GCC also splits assignments to 'volatile' variables and this is actually a bug in gcc.
> 
> volatile unsigned long y;
> 
> y = 0x100000001ul;
> 
>   400728:	c7 05 66 06 20 00 01 	movl   $0x1,0x200666(%rip)        # 600d98 <y>
>   40072f:	00 00 00
>   400732:	c7 05 60 06 20 00 01 	movl   $0x1,0x200660(%rip)        # 600d9c <y+0x4>
>   400739:	00 00 00
> 
> fortunately for y = 0; it generates this:
> 
>   40071d:	48 c7 05 70 06 20 00 	movq   $0x0,0x200670(%rip)        # 600d98 <y>
>   400724:	00 00 00 00
> 
> Thus NULL is safe, but constant ERR_PTR may be dangerous.
> 
> Probably rcu_assign_pointer() should use ACCESS_ONCE() around lvalue, because
> splitting assignment for non-volatile variable seems like completely valid,
> but this may help only after fixing that bug in GCC.

Good catch!  I has queued the following patch.

							Thanx, Paul

------------------------------------------------------------------------

rcu: Add ACCESS_ONCE() to rcu_assign_pointer()

GCC may split assignment to 'long' variable into two instructions:

volatile unsigned long y;

y = 0x100000001ul;

	movl   $0x1,0x200666(%rip)
	movl   $0x1,0x200660(%rip)

This commit fixes this by applying ACCESS_ONCE() within
__rcu_assign_pointer(), but note that some versions and architectures
of GCC have a bug that defeats ACCESS_ONCE():

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981

I added a comment to this bug report asking that the bug be fixed for
volatiles as well as atomics, citing a device driver storing a constant
into a 64-bit device register as motivation.

Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9ed2c9a..3435174 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -556,7 +556,7 @@ static inline void rcu_preempt_sleep_check(void)
 #define __rcu_assign_pointer(p, v, space) \
 	do { \
 		smp_wmb(); \
-		(p) = (typeof(*v) __force space *)(v); \
+		ACCESS_ONCE(p) = (typeof(*v) __force space *)(v); \
 	} while (0)