On Sat, Jun 9, 2018 at 3:20 AM Vladimir Davydov <vdavydov....@gmail.com> 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? thanks, Shakeel
