usb: gadget: f_fs: Ensure io_completion_wq is idle during unbind

During unbind, ffs_func_eps_disable() will be executed, resulting in completion callbacks for any pending USB requests. When using AIO, irrespective of the completion status, io_data work is queued to io_completion_wq to evaluate and handle the completed requests. Since work runs asynchronously to the unbind() routine, there can be a scenario where the work runs after the USB gadget has been fully removed, resulting in accessing of a resource which has been already freed. (i.e. usb_ep_free_request() accessing the USB ep structure)

Explicitly drain the io_completion_wq, instead of relying on the destroy_workqueue() (in ffs_data_put()) to make sure no pending completion work items are running.

---
 drivers/usb/gadget/function/f_fs.c | 3 +++
 1 file changed, 3 insertions(+)

index bf10919..d4844af 100644
@@ -3567,6 +3567,9 @@ static void ffs_func_unbind(struct usb_configuration *c,
 		ffs->func = NULL;
 	}
 
+	/* Drain any pending AIO completions */
+	drain_workqueue(ffs->io_completion_wq);
+
 	if (!--opts->refcnt)
 		functionfs_unbind(ffs);
 

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