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    SubjectRe: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call
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    Sergey Senozhatsky wrote:
    > On (10/29/10 13:16), Paul E. McKenney wrote:
    > > Interesting...
    > >
    > > The task-list lock is read-held at this point, which should mean that
    > > the PID mapping cannot change. The lockdep_tasklist_lock_is_held()
    > > function does lockdep_is_held(&tasklist_lock), which must therefore
    > > only be checking for write-holding the lock. The fix would be to
    > > make lockdep_tasklist_lock_is_held() check for either read-holding or
    > > write-holding tasklist lock.
    > >
    > > Or is there some subtle reason that read-holding the tasklist lock is
    > > not sufficient?

    This was discussed in the thread at http://kerneltrap.org/mailarchive/linux-kernel/2009/12/10/4517520 .
    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.

    And now the patch that adds

    rcu_lockdep_assert(rcu_read_lock_held());

    was merged in accordance with that comment.
    Therefore, I thing below change is not good.

    > Should it be changed to (let's say)
    >
    > struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
    > {
    > - rcu_lockdep_assert(rcu_read_lock_held());
    > + rcu_lockdep_assert(rcu_read_lock_held() || lockdep_tasklist_lock_is_held());
    > return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
    > }

    Regards.


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