On Thu, Sep 17, 2020 at 05:26:44PM +1000, Herbert Xu wrote: > Eric Biggers <ebigg...@kernel.org> wrote: > > From: Eric Biggers <ebigg...@google.com> > > > > When a CPU selects which CRNG to use, it accesses crng_node_pool without > > a memory barrier. That's wrong, because crng_node_pool can be set by > > another CPU concurrently. Without a memory barrier, the crng_state that > > is used might not appear to be fully initialized. > > The only architecture that requires a barrier for data dependency > is Alpha. The correct primitive to ensure that barrier is present > is smp_barrier_depends, or you could just use READ_ONCE. >
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. The last time I tried to use READ_ONCE(), it started a big controversy (https://lkml.kernel.org/linux-fsdevel/20200713033330.205104-1-ebigg...@kernel.org/T/#u, https://lkml.kernel.org/linux-fsdevel/20200717044427.68747-1-ebigg...@kernel.org/T/#u, https://lwn.net/Articles/827180/). In the end, people refused to even allow the READ_ONCE() optimization to be documented, because they felt that smp_load_acquire() should just be used instead. So I think we should just go with smp_load_acquire()... - Eric
