[PATCH v3 1/1] Fix: tracepoint: rcu get state and cond sync for static call updates

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

State transitions from 1->0->1 and N->2->1 callbacks require RCU
synchronization. Rather than performing the RCU synchronization every
time the state change occurs, which is quite slow when many tracepoints
are registered in batch, instead keep a snapshot of the RCU state on the
most recent transitions which belong to a chain, and conditionally wait
for a grace period on the last transition of the chain if one g.p. has
not elapsed since the last snapshot.

This applies to both RCU and SRCU.

Link: https://lore.kernel.org/io-uring/4ebea8f0-58c9-e571-fd30-0ce4f6f09c70@samba.org/
Fixes: <!!!COMMIT ID TBD!!!> ("Fix: tracepoint: static call function vs data state mismatch")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Stefan Metzmacher <metze@samba.org>
Cc: <stable@vger.kernel.org> # 5.10+
---
Changes since v1:
- Use tp_rcu_get_state/tp_rcu_cond_sync on 2->1 transition when
  tp_funcs[0].data != old[0].data rather than
  tracepoint_synchronize_unregister.
Changes since v2:
- Combine duplicated code from add func case TP_FUNC_2 and TP_FUNC_N
  using a fallthrough.
- Add comment.
---
 kernel/tracepoint.c | 81 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 67 insertions(+), 14 deletions(-)

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 8d772bd6894d..efd14c79fab4 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -28,6 +28,44 @@ extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
 DEFINE_SRCU(tracepoint_srcu);
 EXPORT_SYMBOL_GPL(tracepoint_srcu);
 
+enum tp_transition_sync {
+	TP_TRANSITION_SYNC_1_0_1,
+	TP_TRANSITION_SYNC_N_2_1,
+
+	_NR_TP_TRANSITION_SYNC,
+};
+
+struct tp_transition_snapshot {
+	unsigned long rcu;
+	unsigned long srcu;
+	bool ongoing;
+};
+
+/* Protected by tracepoints_mutex */
+static struct tp_transition_snapshot tp_transition_snapshot[_NR_TP_TRANSITION_SYNC];
+
+static void tp_rcu_get_state(enum tp_transition_sync sync)
+{
+	struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
+
+	/* Keep the latest get_state snapshot. */
+	snapshot->rcu = get_state_synchronize_rcu();
+	snapshot->srcu = start_poll_synchronize_srcu(&tracepoint_srcu);
+	snapshot->ongoing = true;
+}
+
+static void tp_rcu_cond_sync(enum tp_transition_sync sync)
+{
+	struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
+
+	if (!snapshot->ongoing)
+		return;
+	cond_synchronize_rcu(snapshot->rcu);
+	if (!poll_state_synchronize_srcu(&tracepoint_srcu, snapshot->srcu))
+		synchronize_srcu(&tracepoint_srcu);
+	snapshot->ongoing = false;
+}
+
 /* Set to 1 to enable tracepoint debug output */
 static const int tracepoint_debug;
 
@@ -311,6 +349,11 @@ static int tracepoint_add_func(struct tracepoint *tp,
 	 */
 	switch (nr_func_state(tp_funcs)) {
 	case TP_FUNC_1:		/* 0->1 */
+		/*
+		 * Make sure new static func never uses old data after a
+		 * 1->0->1 transition sequence.
+		 */
+		tp_rcu_cond_sync(TP_TRANSITION_SYNC_1_0_1);
 		/* Set static call to first function */
 		tracepoint_update_call(tp, tp_funcs);
 		/* Both iterator and static call handle NULL tp->funcs */
@@ -325,10 +368,15 @@ static int tracepoint_add_func(struct tracepoint *tp,
 		 * Requires ordering between RCU assign/dereference and
 		 * static call update/call.
 		 */
-		rcu_assign_pointer(tp->funcs, tp_funcs);
-		break;
+		fallthrough;
 	case TP_FUNC_N:		/* N->N+1 (N>1) */
 		rcu_assign_pointer(tp->funcs, tp_funcs);
+		/*
+		 * Make sure static func never uses incorrect data after a
+		 * N->...->2->1 (N>1) transition sequence.
+		 */
+		if (tp_funcs[0].data != old[0].data)
+			tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
 		break;
 	default:
 		WARN_ON_ONCE(1);
@@ -372,24 +420,23 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 		/* Both iterator and static call handle NULL tp->funcs */
 		rcu_assign_pointer(tp->funcs, NULL);
 		/*
-		 * Make sure new func never uses old data after a 1->0->1
-		 * transition sequence.
-		 * Considering that transition 0->1 is the common case
-		 * and don't have rcu-sync, issue rcu-sync after
-		 * transition 1->0 to break that sequence by waiting for
-		 * readers to be quiescent.
+		 * Make sure new static func never uses old data after a
+		 * 1->0->1 transition sequence.
 		 */
-		tracepoint_synchronize_unregister();
+		tp_rcu_get_state(TP_TRANSITION_SYNC_1_0_1);
 		break;
 	case TP_FUNC_1:		/* 2->1 */
 		rcu_assign_pointer(tp->funcs, tp_funcs);
 		/*
-		 * On 2->1 transition, RCU sync is needed before setting
-		 * static call to first callback, because the observer
-		 * may have loaded any prior tp->funcs after the last one
-		 * associated with an rcu-sync.
+		 * Make sure static func never uses incorrect data after a
+		 * N->...->2->1 (N>2) transition sequence. If the first
+		 * element's data has changed, then force the synchronization
+		 * to prevent current readers that have loaded the old data
+		 * from calling the new function.
 		 */
-		tracepoint_synchronize_unregister();
+		if (tp_funcs[0].data != old[0].data)
+			tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
+		tp_rcu_cond_sync(TP_TRANSITION_SYNC_N_2_1);
 		/* Set static call to first function */
 		tracepoint_update_call(tp, tp_funcs);
 		break;
@@ -397,6 +444,12 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 		fallthrough;
 	case TP_FUNC_N:
 		rcu_assign_pointer(tp->funcs, tp_funcs);
+		/*
+		 * Make sure static func never uses incorrect data after a
+		 * N->...->2->1 (N>2) transition sequence.
+		 */
+		if (tp_funcs[0].data != old[0].data)
+			tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
 		break;
 	default:
 		WARN_ON_ONCE(1);
-- 
2.17.1