On Wed, Oct 23, 2019 at 3:11 PM Christian Brauner <christian.brauner@xxxxxxxxxx> wrote: > > 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. Yes. Not sure about compiler, but hardware definitely can. And generally one does not care if it's compiler or hardware. > 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. No. One puts memory barriers into synchronization primitives. This equally affects memset's, memcpy's and in fact all normal stores. Adding a memory barrier to every normal store is not the solution to this. The memory barrier is done before publication of the memory. And we already have smp_store_release for this. So if one doesn't publish objects with a plain store (which breaks all possible rules anyways) and uses a proper primitive, there is no 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
