Re: [PATCH RFC 06/10] perf tools: lock to protect comm_str rb tree

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Sep 07, 2017 at 10:55:50AM -0700, kan.liang@intel.com escreveu:
> From: Kan Liang <kan.liang@intel.com>
> 
> Add comm_str_lock to protect comm_str rb tree.
> 
> Signed-off-by: Kan Liang <kan.liang@intel.com>
> ---
>  tools/perf/util/comm.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/comm.c b/tools/perf/util/comm.c
> index 7bc981b..1bdfef1 100644
> --- a/tools/perf/util/comm.c
> +++ b/tools/perf/util/comm.c
> @@ -5,6 +5,7 @@
>  #include <stdio.h>
>  #include <string.h>
>  #include <linux/refcount.h>
> +#include <pthread.h>
>  
>  struct comm_str {
>  	char *str;
> @@ -14,6 +15,7 @@ struct comm_str {
>  
>  /* Should perhaps be moved to struct machine */
>  static struct rb_root comm_str_root;
> +static pthread_mutex_t comm_str_lock = PTHREAD_MUTEX_INITIALIZER;
>  
>  static struct comm_str *comm_str__get(struct comm_str *cs)
>  {
> @@ -24,11 +26,13 @@ static struct comm_str *comm_str__get(struct comm_str *cs)
>  
>  static void comm_str__put(struct comm_str *cs)
>  {
> +	pthread_mutex_lock(&comm_str_lock);
>  	if (cs && refcount_dec_and_test(&cs->refcnt)) {
>  		rb_erase(&cs->rb_node, &comm_str_root);
>  		zfree(&cs->str);
>  		free(cs);
>  	}
> +	pthread_mutex_unlock(&comm_str_lock);
>  }

The above should use a smaller locked section, i.e.:

 static void comm_str__put(struct comm_str *cs)
 {
   	if (cs && refcount_dec_and_test(&cs->refcnt)) {
+		pthread_mutex_lock(&comm_str_lock);
   		rb_erase(&cs->rb_node, &comm_str_root);
+		pthread_mutex_unlock(&comm_str_lock);
   		zfree(&cs->str);
   		free(cs);
   	}
 }
  
>  static struct comm_str *comm_str__alloc(const char *str)
> @@ -52,18 +56,22 @@ static struct comm_str *comm_str__alloc(const char *str)
>  
>  static struct comm_str *comm_str__findnew(const char *str, struct rb_root *root)

The usual way is to just rename the above to __comm_str__findnew(),
leaving it unlocked, and then add a locked wrapper:

static struct comm_str *comm_str__findnew(const char *str, struct rb_root *root)
{
	struct comm_str *cs;
	pthread_mutex_lock(&comm_str_lock);
	cs = __comm_str__findnew(str, root);
	pthread_mutex_unlock(&comm_str_lock);
	return cs;
}

>  {
> -	struct rb_node **p = &root->rb_node;
>  	struct rb_node *parent = NULL;
>  	struct comm_str *iter, *new;
> +	struct rb_node **p;
>  	int cmp;
>  
> +	pthread_mutex_lock(&comm_str_lock);
> +	p = &root->rb_node;
>  	while (*p != NULL) {
>  		parent = *p;
>  		iter = rb_entry(parent, struct comm_str, rb_node);
>  
>  		cmp = strcmp(str, iter->str);
> -		if (!cmp)
> -			return comm_str__get(iter);
> +		if (!cmp) {
> +			new = comm_str__get(iter);
> +			goto unlock;
> +		}
>  
>  		if (cmp < 0)
>  			p = &(*p)->rb_left;
> @@ -73,11 +81,13 @@ static struct comm_str *comm_str__findnew(const char *str, struct rb_root *root)
>  
>  	new = comm_str__alloc(str);
>  	if (!new)
> -		return NULL;
> +		goto unlock;
>  
>  	rb_link_node(&new->rb_node, parent, p);
>  	rb_insert_color(&new->rb_node, root);
>  
> +unlock:
> +	pthread_mutex_unlock(&comm_str_lock);
>  	return new;
>  }
>  
> -- 
> 2.5.5