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    SubjectRe: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call
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    Date
    Hello.

    Paul E. McKenney wrote:
    > > Users missing rcu_read_lock() when calling find_task_by_vpid():
    > >
    > > check_clock() in kernel/posix-cpu-timers.c
    >
    > This one has read_lock(&tasklist_lock).
    >
    Excuse me. Holding tasklist_lock lock does not help.
    We must call rcu_read_lock() explicitly.
    That's why 9728e5d6 "kernel/pid.c: update comment on find_task_by_pid_ns" was made.

    I think there are users who needlessly call read_lock(&tasklist_lock)
    when they can use rcu_read_lock() instead.
    But I don't know when to use read_lock(&tasklist_lock).

    If read_lock(&tasklist_lock) is needed only when we want to access
    the "struct task_struct" after rcu_read_unlock(), maybe it is cleaner to
    use a helper like

    struct task_struct *find_task_and_get(pid_t pid)
    {
    struct task_struct *task;
    read_lock(&tasklist_lock);
    rcu_read_lock();
    task = find_task_by_vpid(pid);
    rcu_read_unlock();
    if (task)
    get_task_struct(task);
    read_unlock(&tasklist_lock);
    return task;
    }

    and hide tasklist_lock.

    Regards.


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