On Mon, Sep 21, 2020 at 08:27:14AM -0700, Paul E. McKenney wrote: > On Mon, Sep 21, 2020 at 06:19:39PM +1000, Herbert Xu wrote: > > On Thu, Sep 17, 2020 at 09:58:02AM -0700, Eric Biggers wrote: > > > > > > smp_load_acquire() is obviously correct, whereas READ_ONCE() is an optimization > > > that is difficult to tell whether it's correct or not. For trivial data > > > structures it's "easy" to tell. But whenever there is a->b where b is an > > > internal implementation detail of another kernel subsystem, the use of which > > > could involve accesses to global or static data (for example, spin_lock() > > > accessing lockdep stuff), a control dependency can slip in. > > > > If we're going to follow this line of reasoning, surely you should > > be converting the RCU derference first and foremost, no? ... > And to Eric's point, it is also true that when you have pointers to > static data, and when the compiler can guess this, you do need something > like smp_load_acquire(). But this is a problem only when you are (1) > using feedback-driven compiler optimization or (2) when you compare the > pointer to the address of the static data. Let me restate what I think Eric is saying. He is concerned about the case where a->b and b is some opaque object that may in turn dereference a global data structure unconnected to a. The case in question here is crng_node_pool in drivers/char/random.c which in turn contains a spin lock. But this reasoning could apply to any data structure that contains a spin lock, in particular ones that are dereferenced through RCU. So my question if this reasoning is valid, then why aren't we first converting rcu_dereference to use smp_load_acquire? Cheers, -- Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
