Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 2 Nov 2010 19:33:08 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call |
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> On Tue, 2 Nov 2010, Sergey Senozhatsky wrote: > > > On (11/02/10 16:31), Thomas Gleixner wrote: > > > On Tue, 2 Nov 2010, Sergey Senozhatsky wrote: > > > > > > > Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for > > > > find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns. > > > > Add rcu_read_lock/rcu_read_unlock to call find_task_by_vpid. > > > > > > > > Tetsuo Handa wrote: > > > > > > > > Quoting from one of posts in that thead > > > > http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388 > > > > > > > > | Usually tasklist gives enough protection, but if copy_process() fails > > > > | it calls free_pid() lockless and does call_rcu(delayed_put_pid(). > > > > | This means, without rcu lock find_pid_ns() can't scan the hash table > > > > | safely. > > > > > > We can remove the tasklist_lock while at it. rcu_read_lock is enough. > > >
Yes, I believe posix-cpu-timers.c shouldn't use tasklist at all, but it is not trivial to change this code.
Minor nit,
> > @@ -390,7 +390,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > > > INIT_LIST_HEAD(&new_timer->it.cpu.entry); > > > > - read_lock(&tasklist_lock); > > + rcu_read_lock(); > > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) { > > if (pid == 0) { > > p = current; > > @@ -414,7 +414,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > } else { > > ret = -EINVAL; > > } > > - read_unlock(&tasklist_lock); > > + rcu_read_unlock();
I think this change is fine, but please note that thread_group_leader() check is not relaible without tasklist. If we race with de_thread() find_task_by_vpid() can find the new leader before it updates its ->group_leader. IOW, posix_cpu_timer_create() can fail when it shouldn't.
Not that I think this really matters, posix_cpu_timer_create() has other problems with de_thread(). But perhaps it makes sense to change posix_cpu_timer_create() to use has_group_leader_pid() instead, just to make this code not look racy and avoid adding new problems.
The real fix, I think, should change cpu_timer_list to use struct pid* instead of task_struct.
Oleg.
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