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

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Date:	Tue, 2 Nov 2010 15:58:21 +0200
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect
 find_task_by_vpid call

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.


Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

---

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..855bc53 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
 		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;
 	}
+	rcu_read_unlock();
 	read_unlock(&tasklist_lock);
 
 	return error;
@@ -395,17 +397,21 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
 		if (pid == 0) {
 			p = current;
 		} else {
+			rcu_read_lock();
 			p = find_task_by_vpid(pid);
 			if (p && !same_thread_group(p, current))
 				p = NULL;
+			rcu_read_unlock();
 		}
 	} else {
 		if (pid == 0) {
 			p = current->group_leader;
 		} else {
+			rcu_read_lock();
 			p = find_task_by_vpid(pid);
 			if (p && !thread_group_leader(p))
 				p = NULL;
+			rcu_read_unlock();
 		}
 	}
 	new_timer->it.cpu.task = p;

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