[PATCH] nscd: bump GC cycle during cache pruning (bug 26130)

[Adhemerval Zanella]

On 09/09/2020 11:55, Andreas Schwab wrote:
> While nscd prunes a cache it becomes inconsistent temporarily, which is
> visible to clients if that cache is shared.  Bump the GC cycle counter so
> that the clients notice the modification window.
> 
> Uniformly use atomic_fetch_add to modify the GC cycle counter.
> ---
>  nscd/cache.c | 9 +++++++++
>  nscd/mem.c   | 4 ++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/nscd/cache.c b/nscd/cache.c
> index 94163d9ce3..c0b9b4da3d 100644
> --- a/nscd/cache.c
> +++ b/nscd/cache.c
> @@ -453,6 +453,11 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
>  	  pthread_rwlock_wrlock (&table->lock);
>  	}
>  
> +      /* Now we start modifying the data.  Make sure all readers of the
> +	 data are aware of this and temporarily don't use the data.  */
> +      atomic_fetch_add_relaxed (&table->head->gc_cycle, 1);
> +      assert ((table->head->gc_cycle & 1) == 1);
> +


Is a relaxed atomic suffice here? Other accesses are not done with atomic
at all, so I am not sure how well defined are the 'gc_cycle' usage.

>        while (first <= last)
>  	{
>  	  if (mark[first])
> @@ -493,6 +498,10 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
>  	  ++first;
>  	}
>  
> +      /* Now we are done modifying the data.  */
> +      atomic_fetch_add_relaxed (&table->head->gc_cycle, 1);
> +      assert ((table->head->gc_cycle & 1) == 0);
> +
>        /* It's all done.  */
>        pthread_rwlock_unlock (&table->lock);
>  
> diff --git a/nscd/mem.c b/nscd/mem.c
> index 3d10bb9b46..de5bd12db5 100644
> --- a/nscd/mem.c
> +++ b/nscd/mem.c
> @@ -264,7 +264,7 @@ gc (struct database_dyn *db)
>  
>    /* Now we start modifying the data.  Make sure all readers of the
>       data are aware of this and temporarily don't use the data.  */
> -  ++db->head->gc_cycle;
> +  atomic_fetch_add_relaxed (&db->head->gc_cycle, 1);
>    assert ((db->head->gc_cycle & 1) == 1);
>  
>  
> @@ -490,7 +490,7 @@ gc (struct database_dyn *db)
>  
>  
>    /* Now we are done modifying the data.  */
> -  ++db->head->gc_cycle;
> +  atomic_fetch_add_relaxed (&db->head->gc_cycle, 1);
>    assert ((db->head->gc_cycle & 1) == 0);
>  
>    /* We are done.  */
>