io_uring: add io_aux_cqe which allows deferred completion

Use the just introduced deferred post cqe completion state when possible
in io_aux_cqe. If not possible fallback to io_post_aux_cqe.

This introduces a complication because of allow_overflow. For deferred
completions we cannot know without locking the completion_lock if it will
overflow (and even if we locked it, another post could sneak in and cause
this cqe to be in overflow).
However since overflow protection is mostly a best effort defence in depth
to prevent infinite loops of CQEs for poll, just checking the overflow bit
is going to be good enough and will result in at most 16 (array size of
deferred cqes) overflows.

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Dylan Yudaken <dylany@meta.com>
Link: https://lore.kernel.org/r/20221124093559.3780686-6-dylany@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 34 insertions, 0 deletions

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 28635e3e578a..056aea917cd6 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -830,6 +830,40 @@ bool io_post_aux_cqe(struct io_ring_ctx *ctx,
 	return filled;
 }
 
+bool io_aux_cqe(struct io_ring_ctx *ctx, bool defer, u64 user_data, s32 res, u32 cflags,
+		bool allow_overflow)
+{
+	struct io_uring_cqe *cqe;
+	unsigned int length;
+
+	if (!defer)
+		return io_post_aux_cqe(ctx, user_data, res, cflags, allow_overflow);
+
+	length = ARRAY_SIZE(ctx->submit_state.cqes);
+
+	lockdep_assert_held(&ctx->uring_lock);
+
+	if (ctx->submit_state.cqes_count == length) {
+		io_cq_lock(ctx);
+		__io_flush_post_cqes(ctx);
+		/* no need to flush - flush is deferred */
+		spin_unlock(&ctx->completion_lock);
+	}
+
+	/* For defered completions this is not as strict as it is otherwise,
+	 * however it's main job is to prevent unbounded posted completions,
+	 * and in that it works just as well.
+	 */
+	if (!allow_overflow && test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq))
+		return false;
+
+	cqe = &ctx->submit_state.cqes[ctx->submit_state.cqes_count++];
+	cqe->user_data = user_data;
+	cqe->res = res;
+	cqe->flags = cflags;
+	return true;
+}
+
 static void __io_req_complete_post(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;