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

[Yanqin Wei]

Hi Ilya,

V3 has been updated based on your comments. Thanks.

Best Regards,
Wei Yanqin

> -----Original Message-----
> From: Ilya Maximets <i.maximets at ovn.org>
> Sent: Wednesday, February 26, 2020 9:41 PM
> To: Yanqin Wei <Yanqin.Wei at arm.com>; dev at openvswitch.org
> Cc: nd <nd at arm.com>; Lijian Zhang <Lijian.Zhang at arm.com>; Gavin Hu
> <Gavin.Hu at arm.com>; i.maximets at ovn.org
> Subject: Re: [ovs-dev] [PATCH v2] lib: use acquire-release semantics for pvector
> size
> 
> 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;
> >
> >