linux-kernel - Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by

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Date:	Tue, 2 Nov 2010 18:02:23 +0200
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect
 find_task_by_vpid call

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.
> 

Please kindly review.

---

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..96fe2a3 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -37,13 +37,13 @@ static int check_clock(const clockid_t which_clock)
 	if (pid == 0)
 		return 0;
 
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	p = find_task_by_vpid(pid);
 	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
 		   same_thread_group(p, current) : thread_group_leader(p))) {
 		error = -EINVAL;
 	}
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	return error;
 }
@@ -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();
 
 	return ret;
 }


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