io_uring: export cq overflow status to userspace

For those applications which are not willing to use io_uring_enter()
to reap and handle cqes, they may completely rely on liburing's
io_uring_peek_cqe(), but if cq ring has overflowed, currently because
io_uring_peek_cqe() is not aware of this overflow, it won't enter
kernel to flush cqes, below test program can reveal this bug:

static void test_cq_overflow(struct io_uring *ring)
{
        struct io_uring_cqe *cqe;
        struct io_uring_sqe *sqe;
        int issued = 0;
        int ret = 0;

        do {
                sqe = io_uring_get_sqe(ring);
                if (!sqe) {
                        fprintf(stderr, "get sqe failed\n");
                        break;;
                }
                ret = io_uring_submit(ring);
                if (ret <= 0) {
                        if (ret != -EBUSY)
                                fprintf(stderr, "sqe submit failed: %d\n", ret);
                        break;
                }
                issued++;
        } while (ret > 0);
        assert(ret == -EBUSY);

        printf("issued requests: %d\n", issued);

        while (issued) {
                ret = io_uring_peek_cqe(ring, &cqe);
                if (ret) {
                        if (ret != -EAGAIN) {
                                fprintf(stderr, "peek completion failed: %s\n",
                                        strerror(ret));
                                break;
                        }
                        printf("left requets: %d\n", issued);
                        continue;
                }
                io_uring_cqe_seen(ring, cqe);
                issued--;
                printf("left requets: %d\n", issued);
        }
}

int main(int argc, char *argv[])
{
        int ret;
        struct io_uring ring;

        ret = io_uring_queue_init(16, &ring, 0);
        if (ret) {
                fprintf(stderr, "ring setup failed: %d\n", ret);
                return 1;
        }

        test_cq_overflow(&ring);
        return 0;
}

To fix this issue, export cq overflow status to userspace by adding new
IORING_SQ_CQ_OVERFLOW flag, then helper functions() in liburing, such as
io_uring_peek_cqe, can be aware of this cq overflow and do flush accordingly.
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1274,6 +1274,7 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
 	if (cqe) {
 		clear_bit(0, &ctx->sq_check_overflow);
 		clear_bit(0, &ctx->cq_check_overflow);
+		ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
 	}
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 	io_cqring_ev_posted(ctx);
@@ -1311,6 +1312,7 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
 		if (list_empty(&ctx->cq_overflow_list)) {
 			set_bit(0, &ctx->sq_check_overflow);
 			set_bit(0, &ctx->cq_check_overflow);
+			ctx->rings->sq_flags |= IORING_SQ_CQ_OVERFLOW;
 		}
 		req->flags |= REQ_F_OVERFLOW;
 		refcount_inc(&req->refs);
@@ -6080,9 +6082,9 @@ static int io_sq_thread(void *data)
 			}
 
 			/* Tell userspace we may need a wakeup call */
+			spin_lock_irq(&ctx->completion_lock);
 			ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
-			/* make sure to read SQ tail after writing flags */
-			smp_mb();
+			spin_unlock_irq(&ctx->completion_lock);
 
 			to_submit = io_sqring_entries(ctx);
 			if (!to_submit || ret == -EBUSY) {
@@ -6100,13 +6102,17 @@ static int io_sq_thread(void *data)
 				schedule();
 				finish_wait(&ctx->sqo_wait, &wait);
 
+				spin_lock_irq(&ctx->completion_lock);
 				ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+				spin_unlock_irq(&ctx->completion_lock);
 				ret = 0;
 				continue;
 			}
 			finish_wait(&ctx->sqo_wait, &wait);
 
+			spin_lock_irq(&ctx->completion_lock);
 			ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+			spin_unlock_irq(&ctx->completion_lock);
 		}
 
 		mutex_lock(&ctx->uring_lock);
@@ -7488,6 +7494,7 @@ static void io_uring_cancel_files(struct io_ring_ctx *ctx,
 			if (list_empty(&ctx->cq_overflow_list)) {
 				clear_bit(0, &ctx->sq_check_overflow);
 				clear_bit(0, &ctx->cq_check_overflow);
+				ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
 			}
 			spin_unlock_irq(&ctx->completion_lock);
 

include/uapi/linux/io_uring.h

@@ -197,6 +197,7 @@ struct io_sqring_offsets {
  * sq_ring->flags
  */
 #define IORING_SQ_NEED_WAKEUP	(1U << 0) /* needs io_uring_enter wakeup */
+#define IORING_SQ_CQ_OVERFLOW	(1U << 1) /* CQ ring is overflown */
 
 struct io_cqring_offsets {
 	__u32 head;