lkml.org 
[lkml]   [2010]   [Oct]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call
From
Date
Sergey Senozhatsky wrote:
> On (10/29/10 13:16), Paul E. McKenney wrote:
> > Interesting...
> >
> > The task-list lock is read-held at this point, which should mean that
> > the PID mapping cannot change. The lockdep_tasklist_lock_is_held()
> > function does lockdep_is_held(&tasklist_lock), which must therefore
> > only be checking for write-holding the lock. The fix would be to
> > make lockdep_tasklist_lock_is_held() check for either read-holding or
> > write-holding tasklist lock.
> >
> > Or is there some subtle reason that read-holding the tasklist lock is
> > not sufficient?

This was discussed in the thread at http://kerneltrap.org/mailarchive/linux-kernel/2009/12/10/4517520 .
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.

And now the patch that adds

rcu_lockdep_assert(rcu_read_lock_held());

was merged in accordance with that comment.
Therefore, I thing below change is not good.

> Should it be changed to (let's say)
>
> struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
> {
> - rcu_lockdep_assert(rcu_read_lock_held());
> + rcu_lockdep_assert(rcu_read_lock_held() || lockdep_tasklist_lock_is_held());
> return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
> }

Regards.


\
 
 \ /
  Last update: 2010-10-30 15:17    [from the cache]
©2003-2011 Jasper Spaans