On Sun, Jun 10, 2018 at 9:32 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > On Sun, Jun 10, 2018 at 07:52:50AM -0700, Shakeel Butt wrote: > > On Sat, Jun 9, 2018 at 3:20 AM Vladimir Davydov <vdavydov.dev@xxxxxxxxx> wrote: > > > > > > On Tue, May 29, 2018 at 05:12:04PM -0700, Shakeel Butt wrote: > > > > The memcg kmem cache creation and deactivation (SLUB only) is > > > > asynchronous. If a root kmem cache is destroyed whose memcg cache is in > > > > the process of creation or deactivation, the kernel may crash. > > > > > > > > Example of one such crash: > > > > general protection fault: 0000 [#1] SMP PTI > > > > CPU: 1 PID: 1721 Comm: kworker/14:1 Not tainted 4.17.0-smp > > > > ... > > > > Workqueue: memcg_kmem_cache kmemcg_deactivate_workfn > > > > RIP: 0010:has_cpu_slab > > > > ... > > > > Call Trace: > > > > ? on_each_cpu_cond > > > > __kmem_cache_shrink > > > > kmemcg_cache_deact_after_rcu > > > > kmemcg_deactivate_workfn > > > > process_one_work > > > > worker_thread > > > > kthread > > > > ret_from_fork+0x35/0x40 > > > > > > > > To fix this race, on root kmem cache destruction, mark the cache as > > > > dying and flush the workqueue used for memcg kmem cache creation and > > > > deactivation. > > > > > > > @@ -845,6 +862,8 @@ void kmem_cache_destroy(struct kmem_cache *s) > > > > if (unlikely(!s)) > > > > return; > > > > > > > > + flush_memcg_workqueue(s); > > > > + > > > > > > This should definitely help against async memcg_kmem_cache_create(), > > > but I'm afraid it doesn't eliminate the race with async destruction, > > > unfortunately, because the latter uses call_rcu_sched(): > > > > > > memcg_deactivate_kmem_caches > > > __kmem_cache_deactivate > > > slab_deactivate_memcg_cache_rcu_sched > > > call_rcu_sched > > > kmem_cache_destroy > > > shutdown_memcg_caches > > > shutdown_cache > > > memcg_deactivate_rcufn > > > <dereference destroyed cache> > > > > > > Can we somehow flush those pending rcu requests? > > > > You are right and thanks for catching that. Now I am wondering if > > synchronize_sched() just before flush_workqueue() should be enough. > > Otherwise we might have to replace call_sched_rcu with > > synchronize_sched() in kmemcg_deactivate_workfn which I would not > > prefer as that would holdup the kmem_cache workqueue. > > > > +Paul > > > > Paul, we have a situation something similar to the following pseudo code. > > > > CPU0: > > lock(l) > > if (!flag) > > call_rcu_sched(callback); > > unlock(l) > > ------ > > CPU1: > > lock(l) > > flag = true > > unlock(l) > > synchronize_sched() > > ------ > > > > If CPU0 has called already called call_rchu_sched(callback) then later > > if CPU1 calls synchronize_sched(). Is there any guarantee that on > > return from synchronize_sched(), the rcu callback scheduled by CPU0 > > has already been executed? > > No. There is no such guarantee. > > You instead want rcu_barrier_sched(), which waits for the callbacks from > all prior invocations of call_rcu_sched() to be invoked. > > Please note that synchronize_sched() is -not- sufficient. It is only > guaranteed to wait for a grace period, not necessarily for all prior > callbacks. This goes both directions because if there are no callbacks > in the system, then rcu_barrier_sched() is within its rights to return > immediately. > > So please make sure you use each of synchronize_sched() and > rcu_barrier_sched() to do the job that it was intended to do! ;-) > > If your lock(l) is shorthand for spin_lock(&l), it looks to me like you > actually only need rcu_barrier_sched(): > > CPU0: > spin_lock(&l); > if (!flag) > call_rcu_sched(callback); > spin_unlock(&l); > > CPU1: > spin_lock(&l); > flag = true; > spin_unlock(&l); > /* At this point, no more callbacks will be registered. */ > rcu_barrier_sched(); > /* At this point, all registered callbacks will have been invoked. */ > > On the other hand, if your "lock(l)" was instead shorthand for > rcu_read_lock_sched(), then you need -both- synchronize_sched() -and- > rcu_barrier(). And even then, you will be broken in -rt kernels. > (Which might or might not be a concern, depending on whether your code > matters to -rt kernels. > > Make sense? > Thanks a lot, that was really helpful. The lock is actually mutex_lock. So, I think rcu_barrier_sched() should be sufficient. Shakeel
