On 07/29, Ben Blum wrote: > > According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is > not sufficient to guarantee the tasklist is stable w.r.t. de_thread and > exit. Taking tasklist_lock for reading, instead of rcu_read_lock, > ensures proper exclusion. Yes. So far I still think we should fix while_each_thread() so that it works under rcu_read_lock() "as exepected", I'll try to think more. But whatever we do with while_each_thread(), this can't help cgroup_attach_proc(), it needs the locking. > - rcu_read_lock(); > + read_lock(&tasklist_lock); > if (!thread_group_leader(leader)) { Agreed, this should work. But can't we avoid the global list? thread_group_leader() or not, we do not really care. We only need to ensure we can safely find all threads. How about the patch below? With or without this/your patch this leader can die right after we drop the lock. ss->can_attach(leader) and ss->attach(leader) look suspicious. If a sub-thread execs, this task_struct has nothing to do with the threadgroup. Also. This is off-topic, but... Why cgroup_attach_proc() and cgroup_attach_task() do ->attach_task() + cgroup_task_migrate() in the different order? cgroup_attach_proc() looks wrong even if currently doesn't matter. Oleg. --- x/kernel/cgroup.c +++ x/kernel/cgroup.c @@ -2000,6 +2000,7 @@ int cgroup_attach_proc(struct cgroup *cg /* threadgroup list cursor and array */ struct task_struct *tsk; struct flex_array *group; + unsigned long flags; /* * we need to make sure we have css_sets for all the tasks we're * going to move -before- we actually start moving them, so that in @@ -2027,19 +2028,10 @@ int cgroup_attach_proc(struct cgroup *cg goto out_free_group_list; /* prevent changes to the threadgroup list while we take a snapshot. */ - rcu_read_lock(); - if (!thread_group_leader(leader)) { - /* - * a race with de_thread from another thread's exec() may strip - * us of our leadership, making while_each_thread unsafe to use - * on this task. if this happens, there is no choice but to - * throw this task away and try again (from cgroup_procs_write); - * this is "double-double-toil-and-trouble-check locking". - */ - rcu_read_unlock(); - retval = -EAGAIN; + retval = -EAGAIN; + if (!lock_task_sighand(leader, &flags)) goto out_free_group_list; - } + /* take a reference on each task in the group to go in the array. */ tsk = leader; i = 0; @@ -2055,9 +2047,9 @@ int cgroup_attach_proc(struct cgroup *cg BUG_ON(retval != 0); i++; } while_each_thread(leader, tsk); + unlock_task_sighand(leader, &flags); /* remember the number of threads in the array for later. */ group_size = i; - rcu_read_unlock(); /* * step 1: check that we can legitimately attach to the cgroup. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
