From 0598e4f08e3da1fea2ee3b4765a44798147a8c62 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (VMware)" Date: Thu, 6 Apr 2017 10:28:12 -0400 Subject: ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines The function tracer needs to be more careful than other subsystems when it comes to freeing data. Especially if that data is actually executable code. When a single function is traced, a trampoline can be dynamically allocated which is called to jump to the function trace callback. When the callback is no longer needed, the dynamic allocated trampoline needs to be freed. This is where the issues arise. The dynamically allocated trampoline must not be used again. As function tracing can trace all subsystems, including subsystems that are used to serialize aspects of freeing (namely RCU), it must take extra care when doing the freeing. Before synchronize_rcu_tasks() was around, there was no way for the function tracer to know that nothing was using the dynamically allocated trampoline when CONFIG_PREEMPT was enabled. That's because a task could be indefinitely preempted while sitting on the trampoline. Now with synchronize_rcu_tasks(), it will wait till all tasks have either voluntarily scheduled (not on the trampoline) or goes into userspace (not on the trampoline). Then it is safe to free the trampoline even with CONFIG_PREEMPT set. Acked-by: "Paul E. McKenney" Signed-off-by: Steven Rostedt (VMware) --- kernel/trace/Kconfig | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'kernel/trace/Kconfig') diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig index d4a06e714645..67b463b4f169 100644 --- a/kernel/trace/Kconfig +++ b/kernel/trace/Kconfig @@ -134,7 +134,8 @@ config FUNCTION_TRACER select KALLSYMS select GENERIC_TRACER select CONTEXT_SWITCH_TRACER - select GLOB + select GLOB + select TASKS_RCU if PREEMPT help Enable the kernel to trace every kernel function. This is done by using a compiler feature to insert a small, 5-byte No-Operation -- cgit v1.2.3