[PATCH] Update comment on find_task_by_pid_ns

Tetsuo Handa
2010-02-08T12:30:25Z

Thomas Gleixner wrote:
> On Fri, 11 Dec 2009, Tetsuo Handa wrote:
> > > 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.
> >
> > So, we need to change below comment from "or" to "and" ?
>
> No, both functions must be called with rcu_read_lock()
>
> tasklist_lock read-held is not protecting the rcu lists and does not
> protect against a concurrent update. It merily protects against tasks
> going away or being added while we look up the lists.
>
> > 378 /*
> > 379 * Must be called under rcu_read_lock() or with tasklist_lock read-held.
> > 380 */
> > 381 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
> > 382 {
> > 383 return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
> > 384 }
> > 385
> > 386 struct task_struct *find_task_by_vpid(pid_t vnr)
> > 387 {
> > 388 return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
> > 389 }
> >
>
> Thanks,
>
> tglx

So, we need to update the comment on these functions?
----------------------------------------
[PATCH] Update comment on find_task_by_pid_ns

Caller of find_task_by_vpid() and find_task_by_pid_ns() needs to call
rcu_read_lock() rather than read_lock(&tasklist_lock) because find_pid_ns()
uses RCU primitives but spinlock does not prevent RCU callback if preemptive
RCU ( CONFIG_TREE_PREEMPT_RCU ) is enabled.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
kernel/pid.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-next.orig/kernel/pid.c
+++ linux-next/kernel/pid.c
@@ -376,7 +376,7 @@ struct task_struct *pid_task(struct pid
EXPORT_SYMBOL(pid_task);

/*
- * Must be called under rcu_read_lock() or with tasklist_lock read-held.
+ * Must be called under rcu_read_lock().
*/
struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
{