io_uring/napi: enable even with a timeout of 0

1 usec is not as short as it used to be, and it makes sense to allow 0
for a busy poll timeout - this means just do one loop to check if we
have anything available. Add a separate ->napi_enabled to check if napi
has been enabled or not.

While at it, move the writing of the ctx napi values after we've copied
the old values back to userspace. This ensures that if the call fails,
we'll be in the same state as we were before, rather than some
indeterminate state.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -420,6 +420,7 @@ struct io_ring_ctx {
 	/* napi busy poll default timeout */
 	unsigned int		napi_busy_poll_to;
 	bool			napi_prefer_busy_poll;
+	bool			napi_enabled;
 
 	DECLARE_HASHTABLE(napi_ht, 4);
 #endif

io_uring/napi.c

@@ -227,12 +227,12 @@ int io_register_napi(struct io_ring_ctx *ctx, void __user *arg)
 	if (napi.pad[0] || napi.pad[1] || napi.pad[2] || napi.resv)
 		return -EINVAL;
 
-	WRITE_ONCE(ctx->napi_busy_poll_to, napi.busy_poll_to);
-	WRITE_ONCE(ctx->napi_prefer_busy_poll, !!napi.prefer_busy_poll);
-
 	if (copy_to_user(arg, &curr, sizeof(curr)))
 		return -EFAULT;
 
+	WRITE_ONCE(ctx->napi_busy_poll_to, napi.busy_poll_to);
+	WRITE_ONCE(ctx->napi_prefer_busy_poll, !!napi.prefer_busy_poll);
+	WRITE_ONCE(ctx->napi_enabled, true);
 	return 0;
 }
 
@@ -256,6 +256,7 @@ int io_unregister_napi(struct io_ring_ctx *ctx, void __user *arg)
 
 	WRITE_ONCE(ctx->napi_busy_poll_to, 0);
 	WRITE_ONCE(ctx->napi_prefer_busy_poll, false);
+	WRITE_ONCE(ctx->napi_enabled, false);
 	return 0;
 }
 
@@ -300,7 +301,7 @@ void __io_napi_busy_loop(struct io_ring_ctx *ctx, struct io_wait_queue *iowq)
 {
 	iowq->napi_prefer_busy_poll = READ_ONCE(ctx->napi_prefer_busy_poll);
 
-	if (!(ctx->flags & IORING_SETUP_SQPOLL) && iowq->napi_busy_poll_to)
+	if (!(ctx->flags & IORING_SETUP_SQPOLL) && ctx->napi_enabled)
 		io_napi_blocking_busy_loop(ctx, iowq);
 }