On Fri, 2013-03-29 at 17:12 +0100, Jiri Pirko wrote:
> Fri, Mar 29, 2013 at 04:38:15PM CET, eric.duma...@gmail.com wrote:
> >On Fri, 2013-03-29 at 16:11 +0100, Ivan Vecera wrote:
> >
> >> Erik, why doesn't help the write barrier between the assignments. It
> >> should guarantee their orders... or not?
> >>
> >
> >Its not enough, I wont explain here why as RCU is quite well documented
> >in Documentation/RCU
>
> Can you point me exact paragraph? I'm unable to find that :(
>
You need a bit of RCU history to understand the issue
rcu_assign_pointer(dev->rx_handler, NULL) is certainly not needing a
barrier _before_ setting rx_handler to NULL.
Old kernels had this rcu_assign_pointer() implementation since
commit d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d
(Remove rcu_assign_pointer() penalty for NULL pointers)
#define rcu_assign_pointer(p, v) \
({ \
if (!__builtin_constant_p(v) || \
((v) != NULL)) \
smp_wmb(); \
(p) = (v); \
})
Note that wmb() was _not_ done if v was NULL
Because of various sparse issues, commit
d322f45ceed525daa9401154590bbae3222cfefb
(rcu: Make rcu_assign_pointer() unconditionally insert a memory barrier)
removed the conditional, because RCU_INIT_POINTER() was available.
In the rx_handler/rx_handler_data, we use two pointers protected by RCU,
but we want to only test rx_hander being NULL, and avoid testing
rx_handler_data.
Nothing in RCU guarantees that two different pointers have any order.
Here is what could happen
CPU0 CPU1
handler = rcu_dereference(dev->rx_handler)
if (handler) {
handler(dev, ...);
dev->rx_handler = NULL;
smp_wmb(); // OR NOT
dev->rx_handler_data = NULL;
smp_wmb(); // OR NOT
handler(dev)
priv_data = rcu_dereference(dev->rx_handler_data);
x = priv_data->some_field; // CRASH because priv_data is NULL
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