Eric W. Biederman wrote:
>
> +++ devel-akpm/kernel/task_ref.c 2006-02-27 20:28:59.000000000 -0800
> @@ -0,0 +1,131 @@
> +#include <linux/sched.h>
> +#include <linux/task_ref.h>
> +
> +struct task_ref init_tref = {
> + .count = ATOMIC_INIT(1),
> + .type = PIDTYPE_PID,
> + .pid = 0,
> + .task = NULL,
> +};
Make it static? Actually, I don't understand why init_tref is better
than NULL. Yes, NULL will add some checks into task_ref.c, but we can
avoid some costly atoimic ops.
> +void tref_put(struct task_ref *ref)
> +{
> + might_sleep();
> + if (atomic_dec_and_test(&ref->count)) {
> + struct task_struct *task;
> + BUG_ON(ref == &init_tref);
> + /* Carefully serialize against __detach_pid and tref_get_by_pid */
> + write_lock_irq(&tasklist_lock);
> + task = ref->task;
> + if (task)
> + task->pids[ref->type].ref = NULL;
> + write_unlock_irq(&tasklist_lock);
> + kfree(ref);
> + }
I think this is racy. Suppose ref->count == 1. What if another cpu does
tref_get_by_task() between atomic_dec_and_test() and write_lock_irq() ?
It takes tasklist_lock, increments ->count again, and returns the pointer
to the memory which will be freed soon.
> +struct task_ref *tref_get_by_pid(int pid, enum pid_type type)
> +{
> + struct task_struct *task;
> + struct task_ref *tref;
> +
> + /* Lookup the and pin the task */
> + read_lock(&tasklist_lock);
> + task = find_task_by_pid_type(type, pid);
> + if (task)
> + get_task_struct(task);
> + read_unlock(&tasklist_lock);
> +
> + /* Now get the tref */
> + if (task) {
> + tref = tref_get_by_task(task, type);
> + put_task_struct(task);
> + }
> + else
> + tref = tref_get(&init_tref);
> + return tref;
> +}
I beleive this could be simplified, we don't need to get/put task_struct,
rcu_read_lock();
task = find_task_by_pid_type(type, pid);
if (task)
tref = tref_get_by_task(task, type);
else
tref = tref_get(&init_tref);
rcu_read_unlock();
Oleg.
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