Re: [PATCH] tracing: add trace_event_read_lock()

[Frederic Weisbecker]

On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote:
> 
> I found that there is nothing to protect event_hash in
> ftrace_find_event().



Actually, rcu protects it, but not enough. We have neither
synchronize_rcu() nor rcu_read_lock.

So we protect against concurrent hlist accesses.
But the event can be removed when a module is unloaded,
and that can happen between the time we get the event output
callback and the time we actually use it.

 
> RCU can also protect ftrace_find_event(), but it will add
> latency to kernel. So this fix converts 'trace_event_mutex'
> to a read/write semaphore, and adds trace_event_read_lock()
> to protect ftrace_find_event().
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> ---
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 665a915..4ef8052 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1569,12 +1569,14 @@ static void *s_start(struct seq_file *m, loff_t *pos)
>  		p = s_next(m, p, &l);
>  	}
>  
> +	trace_event_read_lock();
>  	return p;
>  }
>  
>  static void s_stop(struct seq_file *m, void *p)
>  {
>  	atomic_dec(&trace_record_cmdline_disabled);
> +	trace_event_read_unlock();
>  }
>  
>  static void print_lat_help_header(struct seq_file *m)
> @@ -1817,6 +1819,7 @@ static int trace_empty(struct trace_iterator *iter)
>  	return 1;
>  }
>  
> +/*  Called with trace_event_read_lock() held. */
>  static enum print_line_t print_trace_line(struct trace_iterator *iter)
>  {
>  	enum print_line_t ret;
> @@ -3008,6 +3011,7 @@ waitagain:
>  	       offsetof(struct trace_iterator, seq));
>  	iter->pos = -1;
>  
> +	trace_event_read_lock();
>  	while (find_next_entry_inc(iter) != NULL) {
>  		enum print_line_t ret;
>  		int len = iter->seq.len;
> @@ -3024,6 +3028,7 @@ waitagain:
>  		if (iter->seq.len >= cnt)
>  			break;
>  	}
> +	trace_event_read_unlock();
>  	/* Now copy what we have to the user */
>  	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
> @@ -3146,6 +3151,8 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>  		goto out_err;
>  	}
>  
> +	trace_event_read_lock();
> +
>  	/* Fill as many pages as possible. */
>  	for (i = 0, rem = len; i < PIPE_BUFFERS && rem; i++) {
>  		pages[i] = alloc_page(GFP_KERNEL);
> @@ -3168,6 +3175,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>  		trace_seq_init(&iter->seq);
>  	}
>  
> +	trace_event_read_unlock();
>  	mutex_unlock(&iter->mutex);
>  
>  	spd.nr_pages = i;
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 489c0e8..7136420 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -14,7 +14,7 @@
>  /* must be a power of 2 */
>  #define EVENT_HASHSIZE	128
>  
> -static DEFINE_MUTEX(trace_event_mutex);
> +static DECLARE_RWSEM(trace_event_mutex);
>  static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly;
>  
>  static int next_event_type = __TRACE_LAST_TYPE + 1;
> @@ -466,6 +466,7 @@ static int task_state_char(unsigned long state)
>   * @type: the type of event to look for
>   *
>   * Returns an event of type @type otherwise NULL
> + * Called with trace_event_read_lock() held.
>   */
>  struct trace_event *ftrace_find_event(int type)
>  {
> @@ -475,7 +476,7 @@ struct trace_event *ftrace_find_event(int type)
>  
>  	key = type & (EVENT_HASHSIZE - 1);
>  
> -	hlist_for_each_entry_rcu(event, n, &event_hash[key], node) {
> +	hlist_for_each_entry(event, n, &event_hash[key], node) {
>  		if (event->type == type)
>  			return event;
>  	}
> @@ -513,6 +514,16 @@ static int trace_search_list(struct list_head **list)
>  	return last + 1;
>  }
>  
> +void trace_event_read_lock(void)
> +{
> +	down_read(&trace_event_mutex);
> +}
> +
> +void trace_event_read_unlock(void)
> +{
> +	up_read(&trace_event_mutex);
> +}
> +
>  /**
>   * register_ftrace_event - register output for an event type
>   * @event: the event type to register
> @@ -533,7 +544,7 @@ int register_ftrace_event(struct trace_event *event)
>  	unsigned key;
>  	int ret = 0;
>  
> -	mutex_lock(&trace_event_mutex);
> +	down_write(&trace_event_mutex);
>  
>  	if (WARN_ON(!event))
>  		goto out;
> @@ -581,11 +592,11 @@ int register_ftrace_event(struct trace_event *event)
>  
>  	key = event->type & (EVENT_HASHSIZE - 1);
>  
> -	hlist_add_head_rcu(&event->node, &event_hash[key]);
> +	hlist_add_head(&event->node, &event_hash[key]);
>  
>  	ret = event->type;
>   out:
> -	mutex_unlock(&trace_event_mutex);
> +	up_write(&trace_event_mutex);
>  
>  	return ret;
>  }
> @@ -597,10 +608,10 @@ EXPORT_SYMBOL_GPL(register_ftrace_event);
>   */
>  int unregister_ftrace_event(struct trace_event *event)
>  {
> -	mutex_lock(&trace_event_mutex);
> +	down_write(&trace_event_mutex);
>  	hlist_del(&event->node);
>  	list_del(&event->list);
> -	mutex_unlock(&trace_event_mutex);
> +	up_write(&trace_event_mutex);
>  
>  	return 0;
>  }
> diff --git a/kernel/trace/trace_output.h b/kernel/trace/trace_output.h
> index 6e220a8..ac240e7 100644
> --- a/kernel/trace/trace_output.h
> +++ b/kernel/trace/trace_output.h
> @@ -20,6 +20,8 @@ extern int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm,
>  extern int trace_print_context(struct trace_iterator *iter);
>  extern int trace_print_lat_context(struct trace_iterator *iter);
>  
> +extern void trace_event_read_lock(void);
> +extern void trace_event_read_unlock(void);
>  extern struct trace_event *ftrace_find_event(int type);
>  
>  extern enum print_line_t trace_nop_print(struct trace_iterator *iter,



It could be more fine grained. We could have a per event rwsem, and also place the
protected read section only in trace_print_entry() which is the only racy window.

But I'm not sure it's that worthy since event removal is a rare thing.

So I guess this patch is fine.

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