Messages in this thread | | | Date | Fri, 16 Jan 2009 22:48:30 +0100 | From | Louis Rilling <> | Subject | Re: [PATCH 3/3] pids: refactor vnr/nr_ns helpers to make them safe |
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On Fri, Jan 16, 2009 at 09:45:40PM +0100, Oleg Nesterov wrote: > Hi Louis, > > On 01/16, Louis Rilling wrote: > > > > On 16/01/09 6:55 +0100, Oleg Nesterov wrote: > > > + struct pid_namespace *ns) > > > { > > > - return pid_nr_ns(task_pid(tsk), ns); > > > + pid_t nr = 0; > > > + > > > + rcu_read_lock(); > > > + if (!ns) > > > + ns = current->nsproxy->pid_ns; > > > + if (likely(pid_alive(task))) { > > > > I don't see what this pid_alive() check buys you. Since tasklist_lock is not > > enforced, nothing prevents another CPU from detaching the pid right after the > > check. > > pid_alive() should be renamed. We use it to make sure the task didn't pass > __unhash_process(). > > Yes, you are right, nothing prevents another CPU from detaching the pid right > after the check. But this is fine: we read ->pids[].pid under rcu_read_lock(), > and if it is NULL pid_nr_ns() returns. So, we don't need pid_alive() check at > all. > > However, we can not use task->group_leader unless we verify the task is still > alive. That is why we need this check. We do not clear ->group_leader when > the task exits, so we can't do > > rcu_read_lock(); > if (task->group_leader) > do_something(task->group_leader); > rcu_unread_lock(); > > Instead we use pid_alive() before using ->group_leader.
Ok I see now. Since RCU is locked and pid_alive(task) has been true at some point, task->group_leader cannot be freed because release_task() does not release task->group_leader before releasing task. IOW release_task() can't have started releasing task->group_leader before rcu_read_lock().
Thank you for your explanation!
> > > I'm also a bit puzzled by your description with using tasklist_lock when task != > > current, and not seeing tasklist_lock anywhere in the patch. Does this mean that > > "safe" is for "no access to freed memory is done, but caller has to take > > tasklist_lock or may get 0 as return value"? > > I am not sure I understand the question... > > This patch doesn't use tasklist, it relies on rcu. With this patch the caller > doesn't need tasklist/rcu to call these helpers (but of course, the caller > must ensure that task_struct is stable). > > But, whatever the caller does, it can get 0 as return value anyway if the > task exists, this is correct. Or I misunderstood you?
My question was probably badly phrased. When reading the patch description I thought that the point was to fix the helpers so that tasklist_lock would be taken whenever task != current. Of course your patch does not do this, and I'm perfectly fine with this.
Thanks,
Louis
-- Dr Louis Rilling Kerlabs - IRISA Skype: louis.rilling Campus Universitaire de Beaulieu Phone: (+33|0) 2 99 84 71 52 Avenue du General Leclerc Fax: (+33|0) 2 99 84 71 71 35042 Rennes CEDEX - France http://www.kerlabs.com/
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