io_uring: remove ctx->refs pinning on enter

io_uring_enter() takes ctx->refs, which was previously preventing racing
with register quiesce. However, as register now doesn't touch the refs,
we can freely kill extra ctx pinning and rely on the fact that we're
holding a file reference preventing the ring from being destroyed.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a11c57ad33a1be53541fce90669c1b79cf4d8940.1656153286.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -3049,14 +3049,10 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			return -EBADF;
 		ret = -EOPNOTSUPP;
 		if (unlikely(!io_is_uring_fops(f.file)))
-			goto out_fput;
+			goto out;
 	}
 
-	ret = -ENXIO;
 	ctx = f.file->private_data;
-	if (unlikely(!percpu_ref_tryget(&ctx->refs)))
-		goto out_fput;
-
 	ret = -EBADFD;
 	if (unlikely(ctx->flags & IORING_SETUP_R_DISABLED))
 		goto out;
@@ -3141,10 +3137,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 					  &ctx->check_cq);
 		}
 	}
-
 out:
-	percpu_ref_put(&ctx->refs);
-out_fput:
 	fdput(f);
 	return ret;
 }
@@ -3730,11 +3723,10 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 	int ret;
 
 	/*
-	 * We're inside the ring mutex, if the ref is already dying, then
-	 * someone else killed the ctx or is already going through
-	 * io_uring_register().
+	 * We don't quiesce the refs for register anymore and so it can't be
+	 * dying as we're holding a file ref here.
 	 */
-	if (percpu_ref_is_dying(&ctx->refs))
+	if (WARN_ON_ONCE(percpu_ref_is_dying(&ctx->refs)))
 		return -ENXIO;
 
 	if (ctx->restricted) {