Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

[Hannes Frederic Sowa]

On Thu, Oct 10, 2013 at 12:05:32PM -0700, Paul E. McKenney wrote:
> On Thu, Oct 10, 2013 at 04:04:22AM +0200, Hannes Frederic Sowa wrote:
> > On Wed, Oct 09, 2013 at 05:28:33PM -0700, Paul E. McKenney wrote:
> > > On Wed, Oct 09, 2013 at 05:12:40PM -0700, Eric Dumazet wrote:
> > > > On Wed, 2013-10-09 at 16:40 -0700, Josh Triplett wrote:
> > > > 
> > > > > that.  Constructs like list_del_rcu are much clearer, and not
> > > > > open-coded.  Open-coding synchronization code is almost always a Bad
> > > > > Idea.
> > > > 
> > > > OK, so you think there is synchronization code.
> > > > 
> > > > I will shut up then, no need to waste time.
> > > 
> > > As you said earlier, we should at least get rid of the memory barrier
> > > as long as we are changing the code.
> > 
> > Interesting thread!
> > 
> > Sorry to chime in and asking a question:
> > 
> > Why do we need an ACCESS_ONCE here if rcu_assign_pointer can do without one?
> > In other words I wonder why rcu_assign_pointer is not a static inline function
> > to use the sequence point in argument evaluation (if I remember correctly this
> > also holds for inline functions) to not allow something like this:
> > 
> > E.g. we want to publish which lock to take first to prevent an ABBA problem
> > (extreme example):
> > 
> > rcu_assign_pointer(lockptr, min(lptr1, lptr2));
> > 
> > Couldn't a compiler spill the lockptr memory location as a temporary buffer
> > if the compiler is under register pressure? (yes, this seems unlikely if we
> > flushed out most registers to memory because of the barrier, but still... ;) )
> > 
> > This seems to be also the case if we publish a multi-dereferencing pointers
> > e.g. ptr->ptr->ptr.
> 
> IIRC, sequence points only confine volatile accesses.  For non-volatile
> accesses, the so-called "as-if rule" allows compiler writers to do some
> surprisingly global reordering.
> 
> The reason that rcu_assign_pointer() isn't an inline function is because
> it needs to be type-generic, in other words, it needs to be OK to use
> it on any type of pointers as long as the C types of the two pointers
> match (the sparse types can vary a bit).
> 
> One of the reasons for wanting a volatile cast in rcu_assign_pointer() is
> to prevent compiler mischief such as you described in your last two
> paragraphs.  That said, it would take a very brave compiler to pull
> a pointer-referenced memory location into a register and keep it there.
> Unfortunately, increasing compiler bravery seems to be a solid long-term
> trend.

I saw your patch regarding making rcu_assign_pointer volatile and wonder if we
can still make it a bit more safe to use if we force the evaluation of the
to-be-assigned pointer before the write barrier. This is what I have in mind:

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f1f1bc3..79eccc3 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -550,8 +550,9 @@ static inline void rcu_preempt_sleep_check(void)
 	})
 #define __rcu_assign_pointer(p, v, space) \
 	do { \
+		typeof(v) ___v = (v); \
 		smp_wmb(); \
-		(p) = (typeof(*v) __force space *)(v); \
+		(p) = (typeof(*___v) __force space *)(___v); \
 	} while (0)
 
 
I don't think ___v must be volatile for this case because the memory barrier
will force the evaluation of v first.

This would guard against cases where rcu_assign_pointer is used like:

rcu_assign_pointer(ptr, compute_ptr_with_side_effects());

Greetings,

  Hannes

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