On Mon, May 05, 2008 at 01:20:21PM +0200, Nick Piggin wrote:
> While considering the impact of read_barrier_depends, it occurred to
> me that it should really be really a noop for the compiler. At least, it is
> better to have every arch the same than to have a few that are slightly
> different. (Does this mean SMP Alpha's read_barrier_depends could drop the
> "memory" clobber too?)
SMP Alpha's read_barrier_depends() needs the "memory" clobber
because the compiler is otherwise free to move code across the
smp_read_barrier_depends(), which would defeat its purpose.
> --
> It would be a highly unusual compiler that might try to issue a load of
> data1 before it loads a data2 which is data-dependant on data1.
A bit unusual, perhaps, but not unprecedented. Value speculating
compilers, for example.
> There is the problem of the compiler trying to reload data1 _after_
> loading data2, and thus having a newer data1 than data2. However if the
> compiler is so inclined, then it could perform such a load at any point
> after the barrier, so the barrier itself will not guarantee correctness.
>
> I think we've mostly hoped the compiler would not to do that.
Well, this does point me at one thing I missed with preemptable RCU,
namely all the open-coded sequences using smp_read_barrier_depends().
Quite embarrassing!!! But a lot easier having you point me at it than
however long it would have taken me to figure it out on my own, so thank
you very much!!!
> This brings alpha and frv into line with all other architectures.
Assuming that we apply ACCESS_ONCE() as needed to the uses of
smp_read_barrier_depends():
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
>
> Index: linux-2.6/include/asm-alpha/barrier.h
> ===================================================================
> --- linux-2.6.orig/include/asm-alpha/barrier.h
> +++ linux-2.6/include/asm-alpha/barrier.h
> @@ -24,7 +24,7 @@ __asm__ __volatile__("mb": : :"memory")
> #define smp_mb() barrier()
> #define smp_rmb() barrier()
> #define smp_wmb() barrier()
> -#define smp_read_barrier_depends() barrier()
> +#define smp_read_barrier_depends() do { } while (0)
> #endif
>
> #define set_mb(var, value) \
> Index: linux-2.6/include/asm-frv/system.h
> ===================================================================
> --- linux-2.6.orig/include/asm-frv/system.h
> +++ linux-2.6/include/asm-frv/system.h
> @@ -179,7 +179,7 @@ do { \
> #define mb() asm volatile ("membar" : : :"memory")
> #define rmb() asm volatile ("membar" : : :"memory")
> #define wmb() asm volatile ("membar" : : :"memory")
> -#define read_barrier_depends() barrier()
> +#define read_barrier_depends() do { } while (0)
>
> #ifdef CONFIG_SMP
> #define smp_mb() mb()
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