Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

[Steven Rostedt]

On Mon, 10 Feb 2020 12:33:04 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> The rcu_irq_enter/exit_irqson() does atomic_add_return(), which is even worse
> than a memory barrier.

As we discussed on IRC, would something like this work (not even
compiled tested).

-- Steve

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 1fb11daa5c53..a83fd076a312 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -179,10 +179,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		 * For rcuidle callers, use srcu since sched-rcu	\
 		 * doesn't work from the idle path.			\
 		 */							\
-		if (rcuidle) {						\
+		if (rcuidle)						\
 			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
-			rcu_irq_enter_irqson();				\
-		}							\
 									\
 		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
 									\
@@ -194,10 +192,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 			} while ((++it_func_ptr)->func);		\
 		}							\
 									\
-		if (rcuidle) {						\
-			rcu_irq_exit_irqson();				\
+		if (rcuidle)						\
 			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
-		}							\
 									\
 		preempt_enable_notrace();				\
 	} while (0)
diff --git a/include/trace/perf.h b/include/trace/perf.h
index dbc6c74defc3..86d3b2eb00cd 100644
--- a/include/trace/perf.h
+++ b/include/trace/perf.h
@@ -39,17 +39,27 @@ perf_trace_##call(void *__data, proto)					\
 	u64 __count = 1;						\
 	struct task_struct *__task = NULL;				\
 	struct hlist_head *head;					\
+	bool rcu_watching;						\
 	int __entry_size;						\
 	int __data_size;						\
 	int rctx;							\
 									\
+	rcu_watching = rcu_is_watching();				\
+									\
+	/* Can not use RCU if rcu is not watching and in NMI */		\
+	if (!rcu_watching && in_nmi())					\
+		return;							\
+									\
 	__data_size = trace_event_get_offsets_##call(&__data_offsets, args); \
 									\
+	if (!rcu_watching)						\
+		rcu_irq_enter_irqson();					\
+									\
 	head = this_cpu_ptr(event_call->perf_events);			\
 	if (!bpf_prog_array_valid(event_call) &&			\
 	    __builtin_constant_p(!__task) && !__task &&			\
 	    hlist_empty(head))						\
-		return;							\
+		goto out;						\
 									\
 	__entry_size = ALIGN(__data_size + sizeof(*entry) + sizeof(u32),\
 			     sizeof(u64));				\
@@ -57,7 +67,7 @@ perf_trace_##call(void *__data, proto)					\
 									\
 	entry = perf_trace_buf_alloc(__entry_size, &__regs, &rctx);	\
 	if (!entry)							\
-		return;							\
+		goto out;						\
 									\
 	perf_fetch_caller_regs(__regs);					\
 									\
@@ -68,6 +78,9 @@ perf_trace_##call(void *__data, proto)					\
 	perf_trace_run_bpf_submit(entry, __entry_size, rctx,		\
 				  event_call, __count, __regs,		\
 				  head, __task);			\
+out:									\
+	if (!rcu_watching)						\
+		rcu_irq_exit_irqson();					\
 }
 
 /*