On Tue, Sep 22, 2020 at 11:42:43AM -0700, Paul E. McKenney wrote: > On Tue, Sep 22, 2020 at 09:51:36AM +1000, Herbert Xu wrote: > > On Mon, Sep 21, 2020 at 04:26:39PM -0700, Paul E. McKenney wrote: > > > > > > > But this reasoning could apply to any data structure that contains > > > > a spin lock, in particular ones that are dereferenced through RCU. > > > > > > I lost you on this one. What is special about a spin lock? > > > > I don't know, that was Eric's concern. He is inferring that > > spin locks through lockdep debugging may trigger dependencies > > that require smp_load_acquire. > > > > Anyway, my point is if it applies to crng_node_pool then it > > would equally apply to RCU in general. > > Referring to the patch you call out below... > > Huh. The old cmpxchg() primitive is fully ordered, so the old mb() > preceding it must have been for correctly interacting with hardware on > !SMP systems. If that is the case, then the use of cmpxchg_release() > is incorrect. This is not the purview of the memory model, but rather > of device-driver semantics. Or does crng not (or no longer, as the case > might be) interact with hardware RNGs? No hardware involved here. The mb() is just unnecessary, as I noted in my patch https://lore.kernel.org/lkml/20200916233042.51634-1-ebiggers@xxxxxxxxxx/. > What prevents either the old or the new code from kfree()ing the old > state out from under another CPU that just now picked up a pointer to the > old state? The combination of cmpxchg_release() and smp_load_acquire() > won't do anything to prevent this from happening. This is after all not > a memory-ordering issue, but instead an object-lifetime issue. But maybe > you have a lock or something that provides the needed protection. I don't > see how this can be the case and still require the cmpxchg_release() > and smp_load_acquire(), but perhaps this is a failure of imagination on > my part. crng_node_pool is initialized only once, and never freed. - Eric
