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

Paul E. McKenney
2022-06-20T22:52:02Z

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.

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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 90 +++++++++++++++++++++++++++++++++++++++++++++--
kernel/rcu/tree.h | 2 ++
2 files changed, 89 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 46cfceea87847..637e8f9454573 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 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);

@@ -3925,7 +4009,7 @@ 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. */

--
2.31.1.189.g2e36527f23