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 46cfcee..b40a5a1 100644 (file)
--- 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 2ccf584..9c85303 100644 (file)
--- 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. */