[ovs-dev] [PATCH v2] lib: use acquire-release semantics for pvector size

[Ilya Maximets]

On 2/26/20 1:33 AM, Yanqin Wei wrote:
> Read/write concurrency of pvector library is implemented by a temp vector
> and RCU protection. Considering performance reason, insertion does not
> follow this scheme.
> In insertion function, a thread fence ensures size incrementation is done
> after new entry is stored. But there is no barrier in the iteration
> fuction(pvector_cursor_init). Entry point access may be reorderd before
> loading vector size, so the invalid entry point may be loaded when vector
> iteration.
> This patch fixes it by acquire-release pair. It can guarantee new size is
> observed by reader after new entry stored by writer. And this is
> implemented by one-way barrier instead of two-way memory fence.
> 
> Reviewed-by: Gavin Hu <Gavin.Hu at arm.com>
> Reviewed-by: Lijian Zhang <Lijian.Zhang at arm.com>
> Signed-off-by: Yanqin Wei <Yanqin.Wei at arm.com>
> ---
>  lib/pvector.c | 16 +++++++++-------
>  lib/pvector.h | 10 ++++++----
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/lib/pvector.c b/lib/pvector.c
> index aaeee9214..d367079fd 100644
> --- a/lib/pvector.c
> +++ b/lib/pvector.c
> @@ -33,7 +33,7 @@ pvector_impl_alloc(size_t size)
>      struct pvector_impl *impl;
>  
>      impl = xmalloc(sizeof *impl + size * sizeof impl->vector[0]);
> -    impl->size = 0;
> +    atomic_init(&impl->size, 0);
>      impl->allocated = size;
>  
>      return impl;
> @@ -117,18 +117,20 @@ pvector_insert(struct pvector *pvec, void *ptr, int priority)
>  {
>      struct pvector_impl *temp = pvec->temp;
>      struct pvector_impl *old = pvector_impl_get(pvec);
> +    size_t size;
>  
>      ovs_assert(ptr != NULL);
>  
> +    atomic_read_relaxed(&old->size, &size);

I still think that we need a comment here why this read is relaxed.

> +
>      /* Check if can add to the end without reallocation. */
> -    if (!temp && old->allocated > old->size &&
> -        (!old->size || priority <= old->vector[old->size - 1].priority)) {
> -        old->vector[old->size].ptr = ptr;
> -        old->vector[old->size].priority = priority;
> +    if (!temp && old->allocated > size &&
> +        (!size || priority <= old->vector[size - 1].priority)) {
> +        old->vector[size].ptr = ptr;
> +        old->vector[size].priority = priority;
>          /* Size increment must not be visible to the readers before the new
>           * entry is stored. */
> -        atomic_thread_fence(memory_order_release);
> -        ++old->size;
> +        atomic_store_explicit(&old->size, size + 1, memory_order_release);
>      } else {
>          if (!temp) {
>              temp = pvector_impl_dup(old);
> diff --git a/lib/pvector.h b/lib/pvector.h
> index b990ed9d5..c65276638 100644
> --- a/lib/pvector.h
> +++ b/lib/pvector.h
> @@ -69,8 +69,8 @@ struct pvector_entry {
>  };
>  
>  struct pvector_impl {
> -    size_t size;       /* Number of entries in the vector. */
> -    size_t allocated;  /* Number of allocated entries. */
> +    atomic_size_t size;  /* Number of entries in the vector. */
> +    size_t allocated;     /* Number of allocated entries. */

Comment alignment is off.

>      struct pvector_entry vector[];
>  };
>  
> @@ -181,12 +181,14 @@ pvector_cursor_init(const struct pvector *pvec,
>  {
>      const struct pvector_impl *impl;
>      struct pvector_cursor cursor;
> +    size_t size;
>  
>      impl = ovsrcu_get(struct pvector_impl *, &pvec->impl);
>  
> -    ovs_prefetch_range(impl->vector, impl->size * sizeof impl->vector[0]);
> +    atomic_read_explicit(&impl->size, &size, memory_order_acquire);
> +    ovs_prefetch_range(impl->vector, size * sizeof impl->vector[0]);
>  
> -    cursor.size = impl->size;
> +    cursor.size = size;
>      cursor.vector = impl->vector;
>      cursor.entry_idx = -1;
>  
>