[lttng-dev] [PATCH latency-tracker] Fix: use local ops for freelist per-cpu counter

[Julien Desfossez]

Merged, thanks !

Julien

On 23-Aug-2016 11:44:45 PM, Mathieu Desnoyers wrote:
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
>  tracker_private.h     |  3 ++-
>  wrapper/freelist-ll.h | 21 +++++++++++++++------
>  2 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/tracker_private.h b/tracker_private.h
> index 03386b8..fc27f80 100644
> --- a/tracker_private.h
> +++ b/tracker_private.h
> @@ -7,6 +7,7 @@
>  
>  #include <linux/workqueue.h>
>  #include <linux/irq_work.h>
> +#include <asm/local.h>
>  
>  //#include "wrapper/ht.h"
>  //#include "rculfhash-internal.h"
> @@ -18,7 +19,7 @@ struct numa_pool {
>  };
>  
>  struct per_cpu_ll {
> -	int current_count;
> +	local_t current_count;
>  	struct numa_pool *pool;
>  	struct llist_head llist;
>  };
> diff --git a/wrapper/freelist-ll.h b/wrapper/freelist-ll.h
> index 5e4d18e..44db722 100644
> --- a/wrapper/freelist-ll.h
> +++ b/wrapper/freelist-ll.h
> @@ -242,8 +242,8 @@ int free_per_cpu_llist(struct latency_tracker *tracker)
>  		if (!list)
>  			continue;
>  		cnt = free_event_list(list);
> -		printk("freed %d on cpu %d (%d)\n", cnt, cpu,
> -				ll->current_count);
> +		printk("freed %d on cpu %d (%ld)\n", cnt, cpu,
> +				local_read(&ll->current_count));
>  		total_cnt += cnt;
>  	}
>  
> @@ -284,10 +284,14 @@ struct llist_node *per_cpu_get(struct latency_tracker *tracker)
>  	struct per_cpu_ll *ll;
>  
>  	ll = lttng_this_cpu_ptr(tracker->per_cpu_ll);
> +	/*
> +	 * Decrement the current count after we successfully remove an
> +	 * element from the list.
> +	 */
>  	node = llist_del_first(&ll->llist);
>  	if (node) {
> -		ll->current_count--;
> -		WARN_ON_ONCE(ll->current_count < 0);
> +		local_dec(&ll->current_count);
> +		WARN_ON_ONCE(local_read(&ll->current_count) < 0);
>  		return node;
>  	}
>  	return llist_del_first(&ll->pool->llist);
> @@ -336,12 +340,17 @@ void __wrapper_freelist_put_event(struct latency_tracker *tracker,
>  	if (e->pool != ll->pool) {
>  //		printk("DEBUG cross-pool put_event\n");
>  		llist_add(&e->llist, &ll->pool->llist);
> -	} else if (ll->current_count < FREELIST_PERCPU_CACHE) {
> +	} else if (local_read(&ll->current_count) < FREELIST_PERCPU_CACHE) {
>  		/*
>  		 * Fill our local cache if needed.
> +		 * We need to increment current_count before we add the
> +		 * element to the list, because when we successfully
> +		 * remove an element from the list, we expect that the
> +		 * counter is never negative. An interrupt can observe
> +		 * the intermediate state.
>  		 */
> +		local_inc(&ll->current_count);
>  		llist_add(&e->llist, &ll->llist);
> -		ll->current_count++;
>  	} else {
>  		/*
>  		 * Add to our NUMA pool.
> -- 
> 2.1.4
>