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 <khlebni...@openvz.org>
Signed-off-by: Paul E. McKenney <paul...@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)
--
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