Re: [PATCH V7 1/3] tracing: add a possibility of exporting function trace to other places instead of ring buffer only

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 18 Oct 2016 16:08:58 +0800
Chunyan Zhang <zhang.chunyan@linaro.org> wrote:

> Currently Function traces can be only exported to ring buffer, this
> patch added trace_export concept which can process traces and export
> them to a registered destination as an addition to the current only
> one output of Ftrace - i.e. ring buffer.
> 
> In this way, if we want Function traces to be sent to other destination
> rather than ring buffer only, we just need to register a new trace_export
> and implement its own .write() function for writing traces to storage.
> 
> With this patch, only Function trace (trace type is TRACE_FN)
> is supported.

This is getting better, but I still have some nits.

> 
> Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
> ---
>  include/linux/trace.h |  28 +++++++++++
>  kernel/trace/trace.c  | 132 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 159 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/trace.h
> 
> diff --git a/include/linux/trace.h b/include/linux/trace.h
> new file mode 100644
> index 0000000..eb1c5b8
> --- /dev/null
> +++ b/include/linux/trace.h
> @@ -0,0 +1,28 @@
> +#ifndef _LINUX_TRACE_H
> +#define _LINUX_TRACE_H
> +
> +#ifdef CONFIG_TRACING
> +/*
> + * The trace export - an export of Ftrace output. The trace_export
> + * can process traces and export them to a registered destination as
> + * an addition to the current only output of Ftrace - i.e. ring buffer.
> + *
> + * If you want traces to be sent to some other place rather than ring
> + * buffer only, just need to register a new trace_export and implement
> + * its own .write() function for writing traces to the storage.
> + *
> + * next		- pointer to the next trace_export
> + * write	- copy traces which have been delt with ->commit() to
> + *		  the destination
> + */
> +struct trace_export {
> +	struct trace_export __rcu	*next;
> +	void (*write)(const char *, unsigned int);

Why const char*? Why not const void *? This will never be a string.


> +};
> +
> +int register_ftrace_export(struct trace_export *export);
> +int unregister_ftrace_export(struct trace_export *export);
> +
> +#endif	/* CONFIG_TRACING */
> +
> +#endif	/* _LINUX_TRACE_H */
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 8696ce6..db94ec1 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -40,6 +40,7 @@
>  #include <linux/poll.h>
>  #include <linux/nmi.h>
>  #include <linux/fs.h>
> +#include <linux/trace.h>
>  #include <linux/sched/rt.h>
>  
>  #include "trace.h"
> @@ -2128,6 +2129,132 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
>  	ftrace_trace_userstack(buffer, flags, pc);
>  }
>  
> +static void
> +trace_process_export(struct trace_export *export,
> +	       struct ring_buffer_event *event)
> +{
> +	struct trace_entry *entry;
> +	unsigned int size = 0;
> +
> +	entry = ring_buffer_event_data(event);
> +
> +	size = ring_buffer_event_length(event);
> +
> +	if (export->write)
> +		export->write((char *)entry, size);

Is there ever going to be a time where export->write wont be set?

And if there is, this can be racy. As in


	CPU 0:			CPU 1:
	------			------
	if (export->write)

				export->write = NULL;

	export->write(entry, size);

	BOOM!


-- Steve

> +}
> +
> +static DEFINE_MUTEX(ftrace_export_lock);
> +
> +static struct trace_export __rcu *ftrace_exports_list __read_mostly;
> +
> +static DEFINE_STATIC_KEY_FALSE(ftrace_exports_enabled);
> +
> +static inline void ftrace_exports_enable(void)
> +{
> +	static_branch_enable(&ftrace_exports_enabled);
> +}
> +
> +static inline void ftrace_exports_disable(void)
> +{
> +	static_branch_disable(&ftrace_exports_enabled);
> +}
> +
> +void ftrace_exports(struct ring_buffer_event *event)
> +{
> +	struct trace_export *export;
> +
> +	preempt_disable_notrace();
> +
> +	export = rcu_dereference_raw_notrace(ftrace_exports_list);
> +	while (export) {
> +		trace_process_export(export, event);
> +		export = rcu_dereference_raw_notrace(export->next);
> +	}
> +
> +	preempt_enable_notrace();
> +}
> +
> +static inline void
> +add_trace_export(struct trace_export **list, struct trace_export *export)
> +{
> +	rcu_assign_pointer(export->next, *list);
> +	/*
> +	 * We are entering export into the list but another
> +	 * CPU might be walking that list. We need to make sure
> +	 * the export->next pointer is valid before another CPU sees
> +	 * the export pointer included into the list.
> +	 */
> +	rcu_assign_pointer(*list, export);
> +}
> +
> +static inline int
> +rm_trace_export(struct trace_export **list, struct trace_export *export)
> +{
> +	struct trace_export **p;
> +
> +	for (p = list; *p != NULL; p = &(*p)->next)
> +		if (*p == export)
> +			break;
> +
> +	if (*p != export)
> +		return -1;
> +
> +	rcu_assign_pointer(*p, (*p)->next);
> +
> +	return 0;
> +}
> +
> +static inline void
> +add_ftrace_export(struct trace_export **list, struct trace_export *export)
> +{
> +	if (*list == NULL)
> +		ftrace_exports_enable();
> +
> +	add_trace_export(list, export);
> +}
> +
> +static inline int
> +rm_ftrace_export(struct trace_export **list, struct trace_export *export)
> +{
> +	int ret;
> +
> +	ret = rm_trace_export(list, export);
> +	if (*list == NULL)
> +		ftrace_exports_disable();
> +
> +	return ret;
> +}
> +
> +int register_ftrace_export(struct trace_export *export)
> +{
> +	if (WARN_ON_ONCE(!export->write))
> +		return -1;
> +
> +	mutex_lock(&ftrace_export_lock);
> +
> +	add_ftrace_export(&ftrace_exports_list, export);
> +
> +	mutex_unlock(&ftrace_export_lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(register_ftrace_export);
> +
> +int unregister_ftrace_export(struct trace_export *export)
> +{
> +	int ret;
> +
> +	mutex_lock(&ftrace_export_lock);
> +
> +	ret = rm_ftrace_export(&ftrace_exports_list, export);
> +
> +	mutex_unlock(&ftrace_export_lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(unregister_ftrace_export);
> +
>  void
>  trace_function(struct trace_array *tr,
>  	       unsigned long ip, unsigned long parent_ip, unsigned long flags,
> @@ -2146,8 +2273,11 @@ trace_function(struct trace_array *tr,
>  	entry->ip			= ip;
>  	entry->parent_ip		= parent_ip;
>  
> -	if (!call_filter_check_discard(call, entry, buffer, event))
> +	if (!call_filter_check_discard(call, entry, buffer, event)) {
> +		if (static_branch_unlikely(&ftrace_exports_enabled))
> +			ftrace_exports(event);
>  		__buffer_unlock_commit(buffer, event);
> +	}
>  }
>  
>  #ifdef CONFIG_STACKTRACE