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Subject[RFC PATCH v2 06/13] epoll: introduce helpers for adding/removing events to uring
Date
Both add and remove events are lockless and can be called in parallel.

ep_add_event_to_uring():
o user item is marked atomically as ready
o if on previous stem user item was observed as not ready,
then new entry is created for the index uring.

ep_remove_user_item():
o user item is marked as EPOLLREMOVED only if it was ready,
thus userspace will obseve previously added entry in index
uring and correct "removed" state of the item.

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
fs/eventpoll.c | 129 +++++++++++++++++++++++++++++++++
include/uapi/linux/eventpoll.h | 3 +
2 files changed, 132 insertions(+)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 891cc7db8f8d..26d837252ba4 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -189,6 +189,9 @@ struct epitem {

/* Work for offloading event callback */
struct work_struct work;
+
+ /* Bit in user bitmap for user polling */
+ unsigned int bit;
};

/*
@@ -427,6 +430,11 @@ static inline unsigned int ep_to_items_bm_length(unsigned int nr)
return PAGE_ALIGN(ALIGN(nr, 8) >> 3);
}

+static inline unsigned int ep_max_index_nr(struct eventpoll *ep)
+{
+ return ep->index_length >> ilog2(sizeof(*ep->user_index));
+}
+
static inline bool ep_polled_by_user(struct eventpoll *ep)
{
return !!ep->user_header;
@@ -857,6 +865,127 @@ static void epi_rcu_free(struct rcu_head *head)
kmem_cache_free(epi_cache, epi);
}

+#define atomic_set_unless_zero(ptr, flags) \
+({ \
+ typeof(ptr) _ptr = (ptr); \
+ typeof(flags) _flags = (flags); \
+ typeof(*_ptr) _old, _val = READ_ONCE(*_ptr); \
+ \
+ for (;;) { \
+ if (!_val) \
+ break; \
+ _old = cmpxchg(_ptr, _val, _flags); \
+ if (_old == _val) \
+ break; \
+ _val = _old; \
+ } \
+ _val; \
+})
+
+static inline void ep_remove_user_item(struct epitem *epi)
+{
+ struct eventpoll *ep = epi->ep;
+ struct epoll_uitem *uitem;
+
+ lockdep_assert_held(&ep->mtx);
+
+ /* Event should not have any attached queues */
+ WARN_ON(!list_empty(&epi->pwqlist));
+
+ uitem = &ep->user_header->items[epi->bit];
+
+ /*
+ * User item can be in two states: signaled (read_events is set
+ * and userspace has not yet consumed this event) and not signaled
+ * (no events yet fired or already consumed by userspace).
+ * We reset ready_events to EPOLLREMOVED only if ready_events is
+ * in signaled state (we expect that userspace will come soon and
+ * fetch this event). In case of not signaled leave read_events
+ * as 0.
+ *
+ * Why it is important to mark read_events as EPOLLREMOVED in case
+ * of already signaled state? ep_insert() op can be immediately
+ * called after ep_remove(), thus the same bit can be reused and
+ * then new event comes, which corresponds to the same entry inside
+ * user items array. For this particular case ep_add_event_to_uring()
+ * does not allocate a new index entry, but simply masks EPOLLREMOVED,
+ * and userspace uses old index entry, but meanwhile old user item
+ * has been removed, new item has been added and event updated.
+ */
+ atomic_set_unless_zero(&uitem->ready_events, EPOLLREMOVED);
+ clear_bit(epi->bit, ep->items_bm);
+}
+
+#define atomic_or_with_mask(ptr, flags, mask) \
+({ \
+ typeof(ptr) _ptr = (ptr); \
+ typeof(flags) _flags = (flags); \
+ typeof(flags) _mask = (mask); \
+ typeof(*_ptr) _old, _new, _val = READ_ONCE(*_ptr); \
+ \
+ for (;;) { \
+ _new = (_val & ~_mask) | _flags; \
+ _old = cmpxchg(_ptr, _val, _new); \
+ if (_old == _val) \
+ break; \
+ _val = _old; \
+ } \
+ _val; \
+})
+
+static inline bool ep_add_event_to_uring(struct epitem *epi, __poll_t pollflags)
+{
+ struct eventpoll *ep = epi->ep;
+ struct epoll_uitem *uitem;
+ bool added = false;
+
+ if (WARN_ON(!pollflags))
+ return false;
+
+ uitem = &ep->user_header->items[epi->bit];
+ /*
+ * Can be represented as:
+ *
+ * was_ready = uitem->ready_events;
+ * uitem->ready_events &= ~EPOLLREMOVED;
+ * uitem->ready_events |= pollflags;
+ * if (!was_ready) {
+ * // create index entry
+ * }
+ *
+ * See the big comment inside ep_remove_user_item(), why it is
+ * important to mask EPOLLREMOVED.
+ */
+ if (!atomic_or_with_mask(&uitem->ready_events,
+ pollflags, EPOLLREMOVED)) {
+ unsigned int i, *item_idx, index_mask;
+
+ /*
+ * Item was not ready before, thus we have to insert
+ * new index to the ring.
+ */
+
+ index_mask = ep_max_index_nr(ep) - 1;
+ i = __atomic_fetch_add(&ep->user_header->tail, 1,
+ __ATOMIC_ACQUIRE);
+ item_idx = &ep->user_index[i & index_mask];
+
+ /* Signal with a bit, which is > 0 */
+ *item_idx = epi->bit + 1;
+
+ /*
+ * Want index update be flushed from CPU write buffer and
+ * immediately visible on userspace side to avoid long busy
+ * loops.
+ */
+ smp_wmb();
+
+ added = true;
+ }
+
+ return added;
+}
+
/*
* Removes a "struct epitem" from the eventpoll RB tree and deallocates
* all the associated resources. Must be called with "mtx" held.
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index fd06efe5d07e..2008d445b62e 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -42,6 +42,9 @@
#define EPOLLMSG (__force __poll_t)0x00000400
#define EPOLLRDHUP (__force __poll_t)0x00002000

+/* User item marked as removed for EPOLL_USERPOLL */
+#define EPOLLREMOVED ((__force __poll_t)(1U << 27))
+
/* Set exclusive wakeup mode for the target file descriptor */
#define EPOLLEXCLUSIVE ((__force __poll_t)(1U << 28))

--
2.19.1
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