io_uring: kill cached_cq_overflow

There are two copies of cq_overflow, shared with userspace and internal
cached one. It was needed for DRAIN accounting, but now we have yet
another knob to tune the accounting, i.e. cq_extra, and we can throw
away the internal counter and just increment the one in the shared ring.

If user modifies it as so never gets the right overflow value ever
again, it's its problem, even though before we would have restored it
back by next overflow.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8427965f5175dd051febc63804909861109ce859.1621201931.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -364,7 +364,6 @@ struct io_ring_ctx {
 		unsigned		sq_entries;
 		unsigned		sq_thread_idle;
 		unsigned		cached_sq_dropped;
-		unsigned		cached_cq_overflow;
 		unsigned long		sq_check_overflow;
 
 		struct list_head	defer_list;
@@ -1199,13 +1198,20 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	return NULL;
 }
 
+static void io_account_cq_overflow(struct io_ring_ctx *ctx)
+{
+	struct io_rings *r = ctx->rings;
+
+	WRITE_ONCE(r->cq_overflow, READ_ONCE(r->cq_overflow) + 1);
+	ctx->cq_extra--;
+}
+
 static bool req_need_defer(struct io_kiocb *req, u32 seq)
 {
 	if (unlikely(req->flags & REQ_F_IO_DRAIN)) {
 		struct io_ring_ctx *ctx = req->ctx;
 
-		return seq + ctx->cq_extra != ctx->cached_cq_tail
-				+ READ_ONCE(ctx->cached_cq_overflow);
+		return seq + READ_ONCE(ctx->cq_extra) != ctx->cached_cq_tail;
 	}
 
 	return false;
@@ -1444,8 +1450,8 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
 		if (cqe)
 			memcpy(cqe, &ocqe->cqe, sizeof(*cqe));
 		else
-			WRITE_ONCE(ctx->rings->cq_overflow,
-				   ++ctx->cached_cq_overflow);
+			io_account_cq_overflow(ctx);
+
 		posted = true;
 		list_del(&ocqe->list);
 		kfree(ocqe);
@@ -1529,7 +1535,7 @@ static bool io_cqring_event_overflow(struct io_ring_ctx *ctx, u64 user_data,
 		 * or cannot allocate an overflow entry, then we need to drop it
 		 * on the floor.
 		 */
-		WRITE_ONCE(ctx->rings->cq_overflow, ++ctx->cached_cq_overflow);
+		io_account_cq_overflow(ctx);
 		return false;
 	}
 	if (list_empty(&ctx->cq_overflow_list)) {