On Fri, Dec 23, 2011 at 03:27:45AM +0100, Frederic Weisbecker wrote: > On Thu, Dec 22, 2011 at 04:57:51PM -0800, Mandeep Singh Baines wrote: > > Since cgroup_attach_proc is protected by a threadgroup_lock, we > > no longer need a tasklist_lock to protect while_each_thread. > > To keep the complexity of the double-check locking in one place, > > I also moved the thread_group_leader check up into > > attach_task_by_pid. > > > > While at it, also converted a couple of returns to gotos. > > > > The suggestion was made here: > > > > https://lkml.org/lkml/2011/12/22/86 > > > > Suggested-by: Frederic Weisbecker <fweisbec@xxxxxxxxx> > > Signed-off-by: Mandeep Singh Baines <msb@xxxxxxxxxxxx> > > Cc: Tejun Heo <tj@xxxxxxxxxx> > > Cc: Li Zefan <lizf@xxxxxxxxxxxxxx> > > Cc: containers@xxxxxxxxxxxxxxxxxxxxxxxxxx > > Cc: cgroups@xxxxxxxxxxxxxxx > > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Cc: Paul Menage <paul@xxxxxxxxxxxxxx> > > --- > > kernel/cgroup.c | 52 +++++++++++++++++++++------------------------------- > > 1 files changed, 21 insertions(+), 31 deletions(-) > > > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c > > index 1042b3c..032139d 100644 > > --- a/kernel/cgroup.c > > +++ b/kernel/cgroup.c > > @@ -2102,21 +2102,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) > > if (retval) > > goto out_free_group_list; > > > > - /* prevent changes to the threadgroup list while we take a snapshot. */ > > - read_lock(&tasklist_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". > > - */ > > - read_unlock(&tasklist_lock); > > - retval = -EAGAIN; > > - goto out_free_group_list; > > - } > > - > > tsk = leader; > > i = 0; > > do { > > @@ -2145,7 +2130,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) > > group_size = i; > > tset.tc_array = group; > > tset.tc_array_len = group_size; > > - read_unlock(&tasklist_lock); > > You still need rcu_read_lock()/rcu_read_unlock() around > do { > > } while_each_thread() > > because threadgroup_lock() doesn't lock the part that remove a thread from > its group on exit. Actually while_each_thread() takes care of the thread group list safe walking. But we need RCU to ensure the task is not released in parallel. threadgroup_lock() doesn't synchronize against that if the task has already passed the setting of PF_EXITING. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
