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Subject[PATCH 6/6] taskstats: don't use tasklist_lock
Remove tasklist_lock from taskstats.c. find_task_by_pid() is rcu-safe.
->siglock allows us to traverse subthread without tasklist.

Q: delay accounting looks wrong to me. If sub-thread has already
called taskstats_exit_send() but didn't call release_task(self) yet
it will be accounted twice. The window is big. No?

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- STATS/kernel/taskstats.c~6_tasklist 2006-10-27 02:00:32.000000000 +0400
+++ STATS/kernel/taskstats.c 2006-10-27 03:03:40.000000000 +0400
@@ -174,21 +174,19 @@ static void send_cpu_listeners(struct sk
up_write(&listeners->sem);
}

-static int fill_pid(pid_t pid, struct task_struct *pidtsk,
+static int fill_pid(pid_t pid, struct task_struct *tsk,
struct taskstats *stats)
{
int rc = 0;
- struct task_struct *tsk = pidtsk;

- if (!pidtsk) {
- read_lock(&tasklist_lock);
+ if (!tsk) {
+ rcu_read_lock();
tsk = find_task_by_pid(pid);
- if (!tsk) {
- read_unlock(&tasklist_lock);
+ if (tsk)
+ get_task_struct(tsk);
+ rcu_read_unlock();
+ if (!tsk)
return -ESRCH;
- }
- get_task_struct(tsk);
- read_unlock(&tasklist_lock);
} else
get_task_struct(tsk);

@@ -214,40 +212,28 @@ static int fill_pid(pid_t pid, struct ta

}

-static int fill_tgid(pid_t tgid, struct task_struct *tgidtsk,
+static int fill_tgid(pid_t tgid, struct task_struct *first,
struct taskstats *stats)
{
- struct task_struct *tsk, *first;
+ struct task_struct *tsk;
unsigned long flags;
+ int rc = -ESRCH;

/*
* Add additional stats from live tasks except zombie thread group
* leaders who are already counted with the dead tasks
*/
- first = tgidtsk;
- if (!first) {
- read_lock(&tasklist_lock);
+ rcu_read_lock();
+ if (!first)
first = find_task_by_pid(tgid);
- if (!first) {
- read_unlock(&tasklist_lock);
- return -ESRCH;
- }
- get_task_struct(first);
- read_unlock(&tasklist_lock);
- } else
- get_task_struct(first);

+ if (!first || !lock_task_sighand(first, &flags))
+ goto out;

- tsk = first;
- read_lock(&tasklist_lock);
- /* Start with stats from dead tasks */
- if (first->sighand) {
- spin_lock_irqsave(&first->sighand->siglock, flags);
- if (first->signal->stats)
- memcpy(stats, first->signal->stats, sizeof(*stats));
- spin_unlock_irqrestore(&first->sighand->siglock, flags);
- }
+ if (first->signal->stats)
+ memcpy(stats, first->signal->stats, sizeof(*stats));

+ tsk = first;
do {
if (tsk->exit_state == EXIT_ZOMBIE && thread_group_leader(tsk))
continue;
@@ -260,15 +246,18 @@ static int fill_tgid(pid_t tgid, struct
delayacct_add_tsk(stats, tsk);

} while_each_thread(first, tsk);
- read_unlock(&tasklist_lock);
- stats->version = TASKSTATS_VERSION;

+ unlock_task_sighand(first, &flags);
+ rc = 0;
+out:
+ rcu_read_unlock();
+
+ stats->version = TASKSTATS_VERSION;
/*
* Accounting subsytems can also add calls here to modify
* fields of taskstats.
*/
- put_task_struct(first);
- return 0;
+ return rc;
}


-
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