posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call
Sergey Senozhatsky [Wed, 3 Nov 2010 16:52:56 +0000 (18:52 +0200)]
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.

Thomas Gleixner wrote:
| We can remove the tasklist_lock while at it. rcu_read_lock is enough.

Patch also replaces thread_group_leader with has_group_leader_pid
in accordance to comment by Oleg Nesterov:

| ... 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.
|
| 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.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
LKML-Reference: <20101103165256.GD30053@swordfish.minsk.epam.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-cpu-timers.c

index 6842eeb..05bb717 100644 (file)
@@ -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;
 }