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author | Yanqin Wei <Yanqin.Wei@arm.com> | 2020-02-28 00:12:21 +0800 |
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committer | Ilya Maximets <i.maximets@ovn.org> | 2020-03-13 16:29:40 +0100 |
commit | 0df6b0aac057ef039418c2a68e22a519c4e566cd (patch) | |
tree | 07eb6bf84ffb7e1ff9ebd0db8d3e3912d75c7824 | |
parent | f82d11d8f0a92eb85e5d0afb2b2d0a7196256ff7 (diff) | |
download | openvswitch-0df6b0aac057ef039418c2a68e22a519c4e566cd.tar.gz |
pvector: Use acquire-release semantics for size.
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 increment is done
after new entry is stored. But there is no barrier in the iteration
fuction(pvector_cursor_init). Entry point access may be reordered 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.
Fixes: fe7cfa5c3f19 ("lib/pvector: Non-intrusive RCU priority vector.")
Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
-rw-r--r-- | lib/pvector.c | 18 | ||||
-rw-r--r-- | lib/pvector.h | 13 |
2 files changed, 20 insertions, 11 deletions
diff --git a/lib/pvector.c b/lib/pvector.c index fd30819c6..cced388a2 100644 --- a/lib/pvector.c +++ b/lib/pvector.c @@ -29,7 +29,7 @@ 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 +117,22 @@ 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); + /* There is no possible concurrent writer. Insertions must be protected + * by mutex or be always excuted from the same thread. */ + atomic_read_relaxed(&old->size, &size); + /* 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 7e2164ca6..4178eb3e2 100644 --- a/lib/pvector.h +++ b/lib/pvector.h @@ -68,8 +68,8 @@ struct pvector_entry { enum { PVECTOR_EXTRA_ALLOC = 4 }; 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. */ struct pvector_entry vector[]; }; @@ -180,12 +180,17 @@ 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]); + /* Use memory_order_acquire to ensure entry access can not be + * reordered to happen before size read. */ + atomic_read_explicit(&CONST_CAST(struct pvector_impl *, 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; |