Sleeping in RCU list traversal

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Hello.

Peter Zijlstra wrote:
> > Can I sleep between rcu_read_lock() and rcu_read_unlock() ?
> > As far as I saw, rcu_read_lock() makes in_atomic() true, so I think I can't sleep.
> You can indeed not sleep in an rcu_read_lock() section. However, you
> could grab a reference on an item, stop the iteration, drop
> rcu_read_lock. Do you thing, re-acquire rcu_read_lock(), drop the ref,
> and continue the iteration.

Something like this?

  rcu_read_lock();
  list_for_each_rcu(p, ...) {
    ptr = list_entry(p, struct ..., list);
    /* Grab a reference to "ptr". */
    rcu_read_unlock();
    my_task_that_may_sleep(ptr);
    rcu_read_lock();
    /* Drop a reference to "ptr". */
  }
  rcu_read_unlock();

Regarding my case, memory region pointed by "ptr" never be removed.
Do I need to grab a reference to "ptr" ?

Regards.

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