[PATCH v3] io_uring: don't wait when under-submitting

[Pavel Begunkov <asml.silence@gmail.com>]

There is no reliable way to submit and wait in a single syscall, as
io_submit_sqes() may under-consume sqes (in case of an early error).
Then it will wait for not-yet-submitted requests, deadlocking the user
in most cases.

In such cases adjust min_complete, so it won't wait for more than
what have been submitted in the current call to io_uring_enter(). It
may be less than totally in-flight including previous submissions,
but this shouldn't do harm and up to a user.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---

v2: cap min_complete if submitted partially (Jens Axboe)
v3: update commit message (Jens Axboe)

 fs/io_uring.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 81219a631a6d..5dfc805ec31c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3763,11 +3763,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 		unsigned int sqe_flags;
 
 		req = io_get_req(ctx, statep);
-		if (unlikely(!req)) {
-			if (!submitted)
-				submitted = -EAGAIN;
+		if (unlikely(!req))
 			break;
-		}
 		if (!io_get_sqring(ctx, req)) {
 			__io_free_req(req);
 			break;
@@ -5272,6 +5269,14 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 		submitted = io_submit_sqes(ctx, to_submit, f.file, fd,
 					   &cur_mm, false);
 		mutex_unlock(&ctx->uring_lock);
+
+		if (submitted != to_submit) {
+			if (!submitted) {
+				submitted = -EAGAIN;
+				goto done;
+			}
+			min_complete = min(min_complete, (u32)submitted);
+		}
 	}
 	if (flags & IORING_ENTER_GETEVENTS) {
 		unsigned nr_events = 0;
@@ -5284,7 +5289,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			ret = io_cqring_wait(ctx, min_complete, sig, sigsz);
 		}
 	}
-
+done:
 	percpu_ref_put(&ctx->refs);
 out_fput:
 	fdput(f);
-- 
2.24.0