Re: [PATCH rcu 04/12] rcu: Switch polled grace-period APIs to ->gp_seq_polled

Paul E. McKenney
2022-07-22T01:03:50Z

On Wed, Jul 20, 2022 at 10:49:26PM -0700, Boqun Feng wrote:
> On Wed, Jul 20, 2022 at 09:47:08PM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 20, 2022 at 06:51:45PM -0700, Boqun Feng wrote:
> > > On Wed, Jul 20, 2022 at 06:04:55PM -0700, Paul E. McKenney wrote:
> > > [...]
> > > > > > @@ -3860,7 +3944,7 @@ unsigned long get_state_synchronize_rcu(void)
> > > > > > * before the load from ->gp_seq.
> > > > > > */
> > > > > > smp_mb(); /* ^^^ */
> > > > > > - return rcu_seq_snap(&rcu_state.gp_seq);
> > > > > > + return rcu_seq_snap(&rcu_state.gp_seq_polled);
> > > > >
> > > > > I happened to run into this. There is one usage of
> > > > > get_state_synchronize_rcu() in start_poll_synchronize_rcu(), in which
> > > > > the return value of get_state_synchronize_rcu() ("gp_seq") will be used
> > > > > for rcu_start_this_gp(). I don't think this is quite right, because
> > > > > after this change, rcu_state.gp_seq and rcu_state.gp_seq_polled are
> > > > > different values, in fact ->gp_seq_polled is greater than ->gp_seq
> > > > > by how many synchronize_rcu() is called in early boot.
> > > > >
> > > > > Am I missing something here?
> > > >
> > > > It does not appear that your are missing anything, sad to say!
> > > >
> > > > Does the following make it work better?
> > > >
> > > > Thanx, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 2122359f0c862..cf2fd58a93a41 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3571,7 +3571,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
> > > > unsigned long start_poll_synchronize_rcu(void)
> > > > {
> > > > unsigned long flags;
> > > > - unsigned long gp_seq = get_state_synchronize_rcu();
> > > > + unsigned long gp_seq = rcu_seq_snap(&rcu_state.gp_seq);
> > >
> > > get_state_synchronize_rcu() is still needed, because we need to return
> > > a cookie for polling for this function. Something like below maybe? Hope
> > > I didn't mess up the ordering ;-)
> >
> > My thought is to combine your comment with my functionally equivalent
> > code that avoids the extra variable. If that works for you (and if it
> > works, for that matter), does Co-developed-by work for you?
>
> Sure! Thanks ;-)

I did some summarization on the comment, folded it into the original,
and ended up with the patch shown below. Thoughts?

Thanx, Paul

------------------------------------------------------------------------

commit bf95b2bc3e42f11f4d7a5e8a98376c2b4a2aa82f
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Wed Apr 13 17:46:15 2022 -0700

rcu: Switch polled grace-period APIs to ->gp_seq_polled

This commit switches the existing polled grace-period APIs to use a
new ->gp_seq_polled counter in the rcu_state structure. An additional
->gp_seq_polled_snap counter in that same structure allows the normal
grace period kthread to interact properly with the !SMP !PREEMPT fastpath
through synchronize_rcu(). The first of the two to note the end of a
given grace period will make knowledge of this transition available to
the polled API.

This commit is in preparation for polled expedited grace periods.

[ paulmck: Fix use of rcu_state.gp_seq_polled to start normal grace period. ]

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 46cfceea87847..b40a5a19ddd2a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1775,6 +1775,78 @@ static void rcu_strict_gp_boundary(void *unused)
invoke_rcu_core();
}

+// Has rcu_init() been invoked? This is used (for example) to determine
+// whether spinlocks may be acquired safely.
+static bool rcu_init_invoked(void)
+{
+ return !!rcu_state.n_online_cpus;
+}
+
+// Make the polled API aware of the beginning of a grace period.
+static void rcu_poll_gp_seq_start(unsigned long *snap)
+{
+ struct rcu_node *rnp = rcu_get_root();
+
+ if (rcu_init_invoked())
+ raw_lockdep_assert_held_rcu_node(rnp);
+
+ // If RCU was idle, note beginning of GP.
+ if (!rcu_seq_state(rcu_state.gp_seq_polled))
+ rcu_seq_start(&rcu_state.gp_seq_polled);
+
+ // Either way, record current state.
+ *snap = rcu_state.gp_seq_polled;
+}
+
+// Make the polled API aware of the end of a grace period.
+static void rcu_poll_gp_seq_end(unsigned long *snap)
+{
+ struct rcu_node *rnp = rcu_get_root();
+
+ if (rcu_init_invoked())
+ raw_lockdep_assert_held_rcu_node(rnp);
+
+ // If the previously noted GP is still in effect, record the
+ // end of that GP. Either way, zero counter to avoid counter-wrap
+ // problems.
+ if (*snap && *snap == rcu_state.gp_seq_polled) {
+ rcu_seq_end(&rcu_state.gp_seq_polled);
+ rcu_state.gp_seq_polled_snap = 0;
+ } else {
+ *snap = 0;
+ }
+}
+
+// Make the polled API aware of the beginning of a grace period, but
+// where caller does not hold the root rcu_node structure's lock.
+static void rcu_poll_gp_seq_start_unlocked(unsigned long *snap)
+{
+ struct rcu_node *rnp = rcu_get_root();
+
+ if (rcu_init_invoked()) {
+ lockdep_assert_irqs_enabled();
+ raw_spin_lock_irq_rcu_node(rnp);
+ }
+ rcu_poll_gp_seq_start(snap);
+ if (rcu_init_invoked())
+ raw_spin_unlock_irq_rcu_node(rnp);
+}
+
+// Make the polled API aware of the end of a grace period, but where
+// caller does not hold the root rcu_node structure's lock.
+static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
+{
+ struct rcu_node *rnp = rcu_get_root();
+
+ if (rcu_init_invoked()) {
+ lockdep_assert_irqs_enabled();
+ raw_spin_lock_irq_rcu_node(rnp);
+ }
+ rcu_poll_gp_seq_end(snap);
+ if (rcu_init_invoked())
+ raw_spin_unlock_irq_rcu_node(rnp);
+}
+
/*
* Initialize a new grace period. Return false if no grace period required.
*/
@@ -1810,6 +1882,7 @@ static noinline_for_stack bool rcu_gp_init(void)
rcu_seq_start(&rcu_state.gp_seq);
ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
+ rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
raw_spin_unlock_irq_rcu_node(rnp);

/*
@@ -2069,6 +2142,7 @@ static noinline void rcu_gp_cleanup(void)
* safe for us to drop the lock in order to mark the grace
* period as completed in all of the rcu_node structures.
*/
+ rcu_poll_gp_seq_end(&rcu_state.gp_seq_polled_snap);
raw_spin_unlock_irq_rcu_node(rnp);

/*
@@ -3837,8 +3911,18 @@ void synchronize_rcu(void)
lock_is_held(&rcu_lock_map) ||
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu() in RCU read-side critical section");
- if (rcu_blocking_is_gp())
+ if (rcu_blocking_is_gp()) {
+ // Note well that this code runs with !PREEMPT && !SMP.
+ // In addition, all code that advances grace periods runs
+ // at process level. Therefore, this GP overlaps with other
+ // GPs only by being fully nested within them, which allows
+ // reuse of ->gp_seq_polled_snap.
+ rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_snap);
+ rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_snap);
+ if (rcu_init_invoked())
+ cond_resched_tasks_rcu_qs();
return; // Context allows vacuous grace periods.
+ }
if (rcu_gp_is_expedited())
synchronize_rcu_expedited();
else
@@ -3860,7 +3944,7 @@ unsigned long get_state_synchronize_rcu(void)
* before the load from ->gp_seq.
*/
smp_mb(); /* ^^^ */
- return rcu_seq_snap(&rcu_state.gp_seq);
+ return rcu_seq_snap(&rcu_state.gp_seq_polled);
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);

@@ -3889,7 +3973,13 @@ unsigned long start_poll_synchronize_rcu(void)
rdp = this_cpu_ptr(&rcu_data);
rnp = rdp->mynode;
raw_spin_lock_rcu_node(rnp); // irqs already disabled.
- needwake = rcu_start_this_gp(rnp, rdp, gp_seq);
+ // Note it is possible for a grace period to have elapsed between
+ // the above call to get_state_synchronize_rcu() and the below call
+ // to rcu_seq_snap. This is OK, the worst that happens is that we
+ // get a grace period that no one needed. These accesses are ordered
+ // by smp_mb(), and we are accessing them in the opposite order
+ // from which they are updated at grace-period start, as required.
+ needwake = rcu_start_this_gp(rnp, rdp, rcu_seq_snap(&rcu_state.gp_seq));
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
if (needwake)
rcu_gp_kthread_wake();
@@ -3925,7 +4015,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);
bool poll_state_synchronize_rcu(unsigned long oldstate)
{
if (oldstate == RCU_GET_STATE_COMPLETED ||
- rcu_seq_done_exact(&rcu_state.gp_seq, oldstate)) {
+ rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
smp_mb(); /* Ensure GP ends before subsequent accesses. */
return true;
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 2ccf5845957df..9c853033f159d 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -323,6 +323,8 @@ struct rcu_state {
short gp_state; /* GP kthread sleep state. */
unsigned long gp_wake_time; /* Last GP kthread wake. */
unsigned long gp_wake_seq; /* ->gp_seq at ^^^. */
+ unsigned long gp_seq_polled; /* GP seq for polled API. */
+ unsigned long gp_seq_polled_snap; /* ->gp_seq_polled at normal GP start. */

/* End of fields guarded by root rcu_node's lock. */