Re: [PATCH] io_uring: export cq overflow status to userspace

[Jens Axboe <axboe@kernel.dk>]

On 7/7/20 11:29 PM, Xiaoguang Wang wrote:
> I modify above test program a bit:
> #include <errno.h>
> #include <stdio.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <string.h>
> #include <fcntl.h>
> #include <assert.h>
> 
> #include "liburing.h"
> 
> 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;
>          int i;
> 
>          for (i = 0; i < 33; i++) {
>                  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++;
>          }
> 
>          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 %d\n", issued, IO_URING_READ_ONCE(*ring->sq.kflags));
>                          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;
> }
> 
> Though with your patches applied, we still can not peek the last cqe.
> This test program will only issue 33 sqes, so it won't get EBUSY error.

How about we make this even simpler, then - make the
IORING_SQ_CQ_OVERFLOW actually track the state, rather than when we fail
on submission. The liburing change would be the same, the kernel side
would then look like the below.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4c9a494c9f9f..01981926cdf4 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1342,6 +1342,7 @@ 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);
@@ -1379,6 +1380,7 @@ 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);
@@ -6375,9 +6377,9 @@ 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) {
@@ -6400,7 +6402,9 @@ static int io_sq_thread(void *data)
 			}
 			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);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 8d033961cb78..91953b543125 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -198,6 +198,7 @@ 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) /* app needs to enter kernel */
 
 struct io_cqring_offsets {
 	__u32 head;

-- 
Jens Axboe