linux-kernel - [PATCH] Update comment on find_task_by_pid

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Message-Id: <201002082130.JDC57339.OHOVJFQtFSLFMO@I-love.SAKURA.ne.jp>
Date:	Mon, 8 Feb 2010 21:30:17 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	tglx@...utronix.de
Cc:	oleg@...hat.com, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, linux-security-module@...r.kernel.org
Subject: [PATCH] Update comment on find_task_by_pid_ns

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@...ove.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)
 {
--
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