[RFC PATCH 6/6] tracing: use sched-RCU instead of SRCU for rcuidle tracepoints

[Michael Jeanson <mjeanson@efficios.com>]

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Considering that tracer callbacks expect RCU to be watching (for
instance, perf uses rcu_read_lock), we need rcuidle tracepoints to issue
rcu_irq_{enter,exit}_irqson around calls to the callbacks. So there is
no point in using SRCU anymore given that rcuidle tracepoints need to
ensure RCU is watching. Therefore, simply use sched-RCU like normal
tracepoints for rcuidle tracepoints.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michael Jeanson <mjeanson@efficios.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Yonghong Song <yhs@fb.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: bpf@vger.kernel.org
---
 include/linux/tracepoint.h | 33 +++++++--------------------------
 kernel/tracepoint.c        | 25 +++++++++----------------
 2 files changed, 16 insertions(+), 42 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0386b54cbcbb..1414b11f864b 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -13,7 +13,6 @@
  */
 
 #include <linux/smp.h>
-#include <linux/srcu.h>
 #include <linux/errno.h>
 #include <linux/types.h>
 #include <linux/cpumask.h>
@@ -33,8 +32,6 @@ struct trace_eval_map {
 
 #define TRACEPOINT_DEFAULT_PRIO	10
 
-extern struct srcu_struct tracepoint_srcu;
-
 extern int
 tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
 extern int
@@ -86,7 +83,6 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
 static inline void tracepoint_synchronize_unregister(void)
 {
 	synchronize_rcu_tasks_trace();
-	synchronize_srcu(&tracepoint_srcu);
 	synchronize_rcu();
 }
 #else
@@ -175,25 +171,13 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		if (!(cond))						\
 			return;						\
 									\
-		/* srcu can't be used from NMI */			\
-		WARN_ON_ONCE(rcuidle && in_nmi());			\
-									\
-		if (maysleep) {						\
-			might_sleep();					\
+		might_sleep_if(maysleep);				\
+		if (rcuidle)						\
+			rcu_irq_enter_irqson();				\
+		if (maysleep)						\
 			rcu_read_lock_trace();				\
-		} else {						\
-			/* keep srcu and sched-rcu usage consistent */	\
+		else							\
 			preempt_disable_notrace();			\
-		}							\
-									\
-		/*							\
-		 * For rcuidle callers, use srcu since sched-rcu	\
-		 * doesn't work from the idle path.			\
-		 */							\
-		if (rcuidle) {						\
-			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
-			rcu_irq_enter_irqson();				\
-		}							\
 									\
 		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
 									\
@@ -205,15 +189,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 			} while ((++it_func_ptr)->func);		\
 		}							\
 									\
-		if (rcuidle) {						\
-			rcu_irq_exit_irqson();				\
-			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
-		}							\
-									\
 		if (maysleep)						\
 			rcu_read_unlock_trace();			\
 		else							\
 			preempt_enable_notrace();			\
+		if (rcuidle)						\
+			rcu_irq_exit_irqson();				\
 	} while (0)
 
 #ifndef MODULE
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 8d8e41c5d8a5..68b4e50798b1 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -18,9 +18,6 @@
 extern tracepoint_ptr_t __start___tracepoints_ptrs[];
 extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
 
-DEFINE_SRCU(tracepoint_srcu);
-EXPORT_SYMBOL_GPL(tracepoint_srcu);
-
 /* Set to 1 to enable tracepoint debug output */
 static const int tracepoint_debug;
 
@@ -65,14 +62,9 @@ static void rcu_tasks_trace_free_old_probes(struct rcu_head *head)
 	kfree(container_of(head, struct tp_probes, rcu));
 }
 
-static void srcu_free_old_probes(struct rcu_head *head)
-{
-	call_rcu_tasks_trace(head, rcu_tasks_trace_free_old_probes);
-}
-
 static void rcu_free_old_probes(struct rcu_head *head)
 {
-	call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);
+	call_rcu_tasks_trace(head, rcu_tasks_trace_free_old_probes);
 }
 
 static __init int release_early_probes(void)
@@ -90,7 +82,7 @@ static __init int release_early_probes(void)
 	return 0;
 }
 
-/* SRCU and Tasks Trace RCU are initialized at core_initcall */
+/* Tasks Trace RCU is initialized at core_initcall */
 postcore_initcall(release_early_probes);
 
 static inline void release_probes(struct tracepoint_func *old)
@@ -100,9 +92,8 @@ static inline void release_probes(struct tracepoint_func *old)
 			struct tp_probes, probes[0]);
 
 		/*
-		 * We can't free probes if SRCU and Tasks Trace RCU are not
-		 * initialized yet. Postpone the freeing till after both are
-		 * initialized.
+		 * We can't free probes if Tasks Trace RCU is not initialized yet.
+		 * Postpone the freeing till after Tasks Trace RCU is initialized.
 		 */
 		if (unlikely(!ok_to_free_tracepoints)) {
 			tp_probes->rcu.next = early_probes;
@@ -111,9 +102,11 @@ static inline void release_probes(struct tracepoint_func *old)
 		}
 
 		/*
-		 * Tracepoint probes are protected by sched RCU, SRCU and
-		 * Tasks Trace RCU by chaining the callbacks we cover all three
-		 * cases and wait for all three grace periods.
+		 * Tracepoint probes are protected by both sched RCU and
+		 * Tasks Trace RCU, by calling the Tasks Trace RCU callback in
+		 * the sched RCU callback we cover both cases. So let us chain
+		 * the Tasks Trace RCU and sched RCU callbacks to wait for both
+		 * grace periods.
 		 */
 		call_rcu(&tp_probes->rcu, rcu_free_old_probes);
 	}
-- 
2.25.1