On Wed, Oct 23, 2019 at 02:39:55PM +0200, Dmitry Vyukov wrote: > On Wed, Oct 23, 2019 at 2:16 PM Andrea Parri <parri.andrea@xxxxxxxxx> wrote: > > > > On Mon, Oct 21, 2019 at 01:33:27PM +0200, Christian Brauner wrote: > > > When assiging and testing taskstats in taskstats_exit() there's a race > > > when writing and reading sig->stats when a thread-group with more than > > > one thread exits: > > > > > > cpu0: > > > thread catches fatal signal and whole thread-group gets taken down > > > do_exit() > > > do_group_exit() > > > taskstats_exit() > > > taskstats_tgid_alloc() > > > The tasks reads sig->stats without holding sighand lock. > > > > > > cpu1: > > > task calls exit_group() > > > do_exit() > > > do_group_exit() > > > taskstats_exit() > > > taskstats_tgid_alloc() > > > The task takes sighand lock and assigns new stats to sig->stats. > > > > > > The first approach used smp_load_acquire() and smp_store_release(). > > > However, after having discussed this it seems that the data dependency > > > for kmem_cache_alloc() would be fixed by WRITE_ONCE(). > > > Furthermore, the smp_load_acquire() would only manage to order the stats > > > check before the thread_group_empty() check. So it seems just using > > > READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this > > > up for discussion at least. > > > > Mmh, the RELEASE was intended to order the memory initialization in > > kmem_cache_zalloc() with the later ->stats pointer assignment; AFAICT, > > there is no data dependency between such memory accesses. > > I agree. This needs smp_store_release. The latest version that I > looked at contained: > smp_store_release(&sig->stats, stats_new); This is what really makes me wonder. Can the compiler really re-order the kmem_cache_zalloc() call with the assignment. If that's really the case then shouldn't all allocation functions have compiler barriers in them? This then seems like a very generic problem. > > > Correspondingly, the ACQUIRE was intended to order the ->stats pointer > > load with later, _independent dereferences of the same pointer; the > > latter are, e.g., in taskstats_exit() (but not thread_group_empty()). > > How these later loads can be completely independent of the pointer > value? They need to obtain the pointer value from somewhere. And this > can only be done by loaded it. And if a thread loads a pointer and > then dereferences that pointer, that's a data/address dependency and > we assume this is now covered by READ_ONCE. > Or these later loads of the pointer can also race with the store? If To clarify, later loads as in taskstats_exit() and thread_group_empty(), not the later load in the double-checked locking case. > so, I think they also need to use READ_ONCE (rather than turn this earlier > pointer load into acquire). Using READ_ONCE() in the alloc, taskstat_exit(), and thread_group_empty() case. Christian
