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? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
