[PATCH for-next 4/7] io_uring: hide eventfd assumptions in evenfd paths

Pavel Begunkov
2022-06-19T11:26:51Z

Some io_uring-eventfd users assume that there won't be spurious wakeups.
That assumption has to be honoured by all io_cqring_ev_posted() callers,
which is inconvenient and from time to time leads to problems but should
be maintained to not break the userspace.

Instead of making the callers to track whether a CQE was posted or not,
hide it inside io_eventfd_signal(). It saves ->cached_cq_tail it saw
last time and triggers the eventfd only when ->cached_cq_tail changed
since then.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
include/linux/io_uring_types.h | 2 ++
io_uring/io_uring.c | 44 ++++++++++++++++++++--------------
io_uring/timeout.c | 3 +--
3 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 779c72da5b8f..327bc7f0808d 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -315,6 +315,8 @@ struct io_ring_ctx {

struct list_head defer_list;
unsigned sq_thread_idle;
+ /* protected by ->completion_lock */
+ unsigned evfd_last_cq_tail;
};

enum {
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 31beb9ccbf12..0875cc649e23 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -473,6 +473,22 @@ static __cold void io_queue_deferred(struct io_ring_ctx *ctx)
static void io_eventfd_signal(struct io_ring_ctx *ctx)
{
struct io_ev_fd *ev_fd;
+ bool skip;
+
+ spin_lock(&ctx->completion_lock);
+ /*
+ * Eventfd should only get triggered when at least one event has been
+ * posted. Some applications rely on the eventfd notification count only
+ * changing IFF a new CQE has been added to the CQ ring. There's no
+ * depedency on 1:1 relationship between how many times this function is
+ * called (and hence the eventfd count) and number of CQEs posted to the
+ * CQ ring.
+ */
+ skip = ctx->cached_cq_tail == ctx->evfd_last_cq_tail;
+ ctx->evfd_last_cq_tail = ctx->cached_cq_tail;
+ spin_unlock(&ctx->completion_lock);
+ if (skip)
+ return;

rcu_read_lock();
/*
@@ -511,13 +527,6 @@ void __io_commit_cqring_flush(struct io_ring_ctx *ctx)
io_eventfd_signal(ctx);
}

-/*
- * This should only get called when at least one event has been posted.
- * Some applications rely on the eventfd notification count only changing
- * IFF a new CQE has been added to the CQ ring. There's no depedency on
- * 1:1 relationship between how many times this function is called (and
- * hence the eventfd count) and number of CQEs posted to the CQ ring.
- */
void io_cqring_ev_posted(struct io_ring_ctx *ctx)
{
if (unlikely(ctx->off_timeout_used || ctx->drain_active ||
@@ -530,7 +539,7 @@ void io_cqring_ev_posted(struct io_ring_ctx *ctx)
/* Returns true if there are no backlogged entries after the flush */
static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
{
- bool all_flushed, posted;
+ bool all_flushed;
size_t cqe_size = sizeof(struct io_uring_cqe);

if (!force && __io_cqring_events(ctx) == ctx->cq_entries)
@@ -539,7 +548,6 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
if (ctx->flags & IORING_SETUP_CQE32)
cqe_size <<= 1;

- posted = false;
spin_lock(&ctx->completion_lock);
while (!list_empty(&ctx->cq_overflow_list)) {
struct io_uring_cqe *cqe = io_get_cqe(ctx);
@@ -554,7 +562,6 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
else
io_account_cq_overflow(ctx);

- posted = true;
list_del(&ocqe->list);
kfree(ocqe);
}
@@ -567,8 +574,7 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)

io_commit_cqring(ctx);
spin_unlock(&ctx->completion_lock);
- if (posted)
- io_cqring_ev_posted(ctx);
+ io_cqring_ev_posted(ctx);
return all_flushed;
}

@@ -758,8 +764,7 @@ bool io_post_aux_cqe(struct io_ring_ctx *ctx,
filled = io_fill_cqe_aux(ctx, user_data, res, cflags);
io_commit_cqring(ctx);
spin_unlock(&ctx->completion_lock);
- if (filled)
- io_cqring_ev_posted(ctx);
+ io_cqring_ev_posted(ctx);
return filled;
}

@@ -940,14 +945,12 @@ __cold void io_free_req(struct io_kiocb *req)
static void __io_req_find_next_prep(struct io_kiocb *req)
{
struct io_ring_ctx *ctx = req->ctx;
- bool posted;

spin_lock(&ctx->completion_lock);
- posted = io_disarm_next(req);
+ io_disarm_next(req);
io_commit_cqring(ctx);
spin_unlock(&ctx->completion_lock);
- if (posted)
- io_cqring_ev_posted(ctx);
+ io_cqring_ev_posted(ctx);
}

static inline struct io_kiocb *io_req_find_next(struct io_kiocb *req)
@@ -2431,6 +2434,11 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
kfree(ev_fd);
return ret;
}
+
+ spin_lock(&ctx->completion_lock);
+ ctx->evfd_last_cq_tail = ctx->cached_cq_tail;
+ spin_unlock(&ctx->completion_lock);
+
ev_fd->eventfd_async = eventfd_async;
ctx->has_evfd = true;
rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
diff --git a/io_uring/timeout.c b/io_uring/timeout.c
index 557c637af158..4938c1cdcbcd 100644
--- a/io_uring/timeout.c
+++ b/io_uring/timeout.c
@@ -628,7 +628,6 @@ __cold bool io_kill_timeouts(struct io_ring_ctx *ctx, struct task_struct *tsk,
spin_unlock_irq(&ctx->timeout_lock);
io_commit_cqring(ctx);
spin_unlock(&ctx->completion_lock);
- if (canceled != 0)
- io_cqring_ev_posted(ctx);
+ io_cqring_ev_posted(ctx);
return canceled != 0;
}
--
2.36.1