On Fri, Mar 19, 2021 at 02:58:54PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> >
> > There is a need for a non-blocking polling interface for RCU grace
> > periods, so this commit supplies start_poll_synchronize_rcu() and
> > poll_state_synchronize_rcu() for this purpose. Note that the existing
> > get_state_synchronize_rcu() may be used if future grace periods are
> > inevitable (perhaps due to a later call_rcu() invocation). The new
> > start_poll_synchronize_rcu() is to be used if future grace periods
> > might not otherwise happen. Finally, poll_state_synchronize_rcu()
> > provides a lockless check for a grace period having elapsed since
> > the corresponding call to either of the get_state_synchronize_rcu()
> > or start_poll_synchronize_rcu().
> >
> > As with get_state_synchronize_rcu(), the return value from either
> > get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
> > to a later call to either poll_state_synchronize_rcu() or the existing
> > (might_sleep) cond_synchronize_rcu().
>
> It's all a matter of personal taste but if I may suggest some namespace
> modifications:
>
> get_state_synchronize_rcu() -> synchronize_rcu_poll_start_raw()
> start_poll_synchronize_rcu() -> synchronize_rcu_poll_start()
> poll_state_synchronize_rcu() -> synchronize_rcu_poll()
> cond_synchronize_rcu() -> synchronize_rcu_cond()
>
> But it's up to you really.
I am concerned about starting anything "synchronize_rcu" if that
thing doesn't unconditionally wait for a grace period. "What do
you mean that there was no grace period? Don't you see that call to
synchronize_rcu_poll_start_raw()???"
This objection doesn't apply to cond_synchronize_rcu(), but it is
already in use, so any name change should be worked with the users.
All two of them. ;-)
> > /**
> > + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period
> > + *
> > + * Returns a cookie that is used by a later call to cond_synchronize_rcu()
>
> It may be worth noting that calling start_poll_synchronize_rcu() and then
> pass the cookie to cond_synchronize_rcu() soon after may end up waiting for
> one more grace period.
You mean this sequence of events?
1. cookie = start_poll_synchronize_rcu()
2. The grace period corresponding to cookie is almost over...
3. cond_synchronize_rcu() checks the cookie and sees that the
grace period has not yet expired.
4. The grace period corresponding to cookie completes.
5. Someone else starts a grace period.
6. cond_synchronize_rcu() invokes synchronize_rcu(), which waits
for the just-started grace period plus another grace period.
Thus, there has been no fewer than three full grace periods
between the call to start_poll_synchronize_rcu() and the
return from cond_synchronize_rcu().
Yes, this can happen! And it can be worse, for example, it is quite
possible that cond_synchronize_rcu() would be preempted for multiple
grace periods at step 5, in which case it would still wait for almost
two additional grace periods.
Or are you thinking of something else?
> > + * or poll_state_synchronize_rcu() to determine whether or not a full
> > + * grace period has elapsed in the meantime. If the needed grace period
> > + * is not already slated to start, notifies RCU core of the need for that
> > + * grace period.
> > + *
> > + * Interrupts must be enabled for the case where it is necessary to awaken
> > + * the grace-period kthread.
> > + */
> > +unsigned long start_poll_synchronize_rcu(void)
> > +{
> > + unsigned long flags;
> > + unsigned long gp_seq = get_state_synchronize_rcu();
> > + bool needwake;
> > + struct rcu_data *rdp;
> > + struct rcu_node *rnp;
> [...]
> > +
> > +/**
> > + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
> > + *
> > + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
> > + *
> > + * If a full RCU grace period has elapsed since the earlier call from
> > + * which oldstate was obtained, return @true, otherwise return @false.
> > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
>
> Rephrase suggestion for the last sentence:
>
> "In case of failure, it's up to the caller to try polling again later or
> invoke synchronize_rcu() to wait for a new full grace period to complete."
How about like this?
/**
* poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
*
* @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
*
* If a full RCU grace period has elapsed since the earlier call from
* which oldstate was obtained, return @true, otherwise return @false.
* If @false is returned, it is the caller's responsibilty to invoke this
* function later on until it does return @true. Alternatively, the caller
* can explicitly wait for a grace period, for example, by passing @oldstate
* to cond_synchronize_rcu() or by directly invoking synchronize_rcu().
*
* Yes, this function does not take counter wrap into account.
* But counter wrap is harmless. If the counter wraps, we have waited for
* more than 2 billion grace periods (and way more on a 64-bit system!).
* Those needing to keep oldstate values for very long time periods
* (many hours even on 32-bit systems) should check them occasionally
* and either refresh them or set a flag indicating that the grace period
* has completed.
*/
> In any case: Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Thank you, I will apply it at the next rebase.
Thanx, Paul
