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

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Date:	Wed, 3 Nov 2010 12:58:32 +0200
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect
 find_task_by_vpid call

On (11/02/10 19:33), Oleg Nesterov wrote:
> > On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:
> >
> > > > We can remove the tasklist_lock while at it. rcu_read_lock is enough.
> > > >
> 
> Yes, I believe posix-cpu-timers.c shouldn't use tasklist at all,
> but it is not trivial to change this code.
>
>[..] 
> 
> I think this change is fine, but please note that thread_group_leader()
> check is not relaible without tasklist. If we race with de_thread()
> find_task_by_vpid() can find the new leader before it updates its
> ->group_leader. IOW, posix_cpu_timer_create() can fail when it shouldn't.
> 
> Not that I think this really matters, posix_cpu_timer_create() has
> other problems with de_thread(). But perhaps it makes sense to
> change posix_cpu_timer_create() to use has_group_leader_pid() instead,
> just to make this code not look racy and avoid adding new problems.
> 
> The real fix, I think, should change cpu_timer_list to use
> struct pid* instead of task_struct.
> 

Hello,
Using has_group_leader_pid instead of thread_group_leader, when tasklist_lock 
is not aquired (check_clock and posix_cpu_timer_create).

---

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..05bb717 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))) {
+		   same_thread_group(p, current) : has_group_leader_pid(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;
@@ -404,7 +404,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
 			p = current->group_leader;
 		} else {
 			p = find_task_by_vpid(pid);
-			if (p && !thread_group_leader(p))
+			if (p && !has_group_leader_pid(p))
 				p = NULL;
 		}
 	}
@@ -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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