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

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Paul E. McKenney wrote:
> 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)

Seems like RCU_INIT_POINTER() need this too.

>
>
>
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