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Message-Id: <20210805192954.30688-1-mathieu.desnoyers@efficios.com>
Date: Thu, 5 Aug 2021 15:29:54 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: rostedt@...dmis.org
Cc: linux-kernel@...r.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Stefan Metzmacher <metze@...ba.org>, stable@...r.kernel.org
Subject: [PATCH v3 1/1] Fix: tracepoint: rcu get state and cond sync for static call updates
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@...icios.com>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Stefan Metzmacher <metze@...ba.org>
Cc: <stable@...r.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
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