Re: [PATCH 1/4] ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 06, 2017 at 12:42:38PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> 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.
> 
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

One question below.  Other than that:

Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  kernel/trace/Kconfig  |  3 ++-
>  kernel/trace/ftrace.c | 42 ++++++++++++++++++------------------------
>  2 files changed, 20 insertions(+), 25 deletions(-)
> 
> 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

Does GLOB really want to be selected in production environments?
I could understand "select GLOB if WE_ARE_TESTING" or some such.

> +	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
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 8efd9fe7aec0..34f63e78d661 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2808,18 +2808,28 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
>  	 * callers are done before leaving this function.
>  	 * The same goes for freeing the per_cpu data of the per_cpu
>  	 * ops.
> -	 *
> -	 * Again, normal synchronize_sched() is not good enough.
> -	 * We need to do a hard force of sched synchronization.
> -	 * This is because we use preempt_disable() to do RCU, but
> -	 * the function tracers can be called where RCU is not watching
> -	 * (like before user_exit()). We can not rely on the RCU
> -	 * infrastructure to do the synchronization, thus we must do it
> -	 * ourselves.
>  	 */
>  	if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_PER_CPU)) {
> +		/*
> +		 * We need to do a hard force of sched synchronization.
> +		 * This is because we use preempt_disable() to do RCU, but
> +		 * the function tracers can be called where RCU is not watching
> +		 * (like before user_exit()). We can not rely on the RCU
> +		 * infrastructure to do the synchronization, thus we must do it
> +		 * ourselves.
> +		 */
>  		schedule_on_each_cpu(ftrace_sync);
> 
> +		/*
> +		 * When the kernel is preeptive, tasks can be preempted
> +		 * while on a ftrace trampoline. Just scheduling a task on
> +		 * a CPU is not good enough to flush them. Calling
> +		 * synchornize_rcu_tasks() will wait for those tasks to
> +		 * execute and either schedule voluntarily or enter user space.
> +		 */
> +		if (IS_ENABLED(CONFIG_PREEMPT))
> +			synchronize_rcu_tasks();
> +
>  		arch_ftrace_trampoline_free(ops);
> 
>  		if (ops->flags & FTRACE_OPS_FL_PER_CPU)
> @@ -5366,22 +5376,6 @@ void __weak arch_ftrace_update_trampoline(struct ftrace_ops *ops)
> 
>  static void ftrace_update_trampoline(struct ftrace_ops *ops)
>  {
> -
> -/*
> - * Currently there's no safe way to free a trampoline when the kernel
> - * is configured with PREEMPT. That is because a task could be preempted
> - * when it jumped to the trampoline, it may be preempted for a long time
> - * depending on the system load, and currently there's no way to know
> - * when it will be off the trampoline. If the trampoline is freed
> - * too early, when the task runs again, it will be executing on freed
> - * memory and crash.
> - */
> -#ifdef CONFIG_PREEMPT
> -	/* Currently, only non dynamic ops can have a trampoline */
> -	if (ops->flags & FTRACE_OPS_FL_DYNAMIC)
> -		return;
> -#endif
> -
>  	arch_ftrace_update_trampoline(ops);
>  }
> 
> -- 
> 2.10.2
> 
>