Messages in this thread | | | Date | Thu, 10 Oct 2013 12:05:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive |
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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.
Thanx, Paul
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