From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A429C33CB1 for ; Tue, 14 Jan 2020 14:41:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C96FB2467D for ; Tue, 14 Jan 2020 14:41:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728741AbgANOln (ORCPT ); Tue, 14 Jan 2020 09:41:43 -0500 Received: from mx2.suse.de ([195.135.220.15]:56086 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725904AbgANOln (ORCPT ); Tue, 14 Jan 2020 09:41:43 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 3E417BED6; Tue, 14 Jan 2020 14:41:41 +0000 (UTC) Message-ID: <1579012899.15925.7.camel@suse.com> Subject: Re: [PATCH] usb: uas: fix a plug & unplug racing From: Oliver Neukum To: EJ Hsu Cc: "linux-usb@vger.kernel.org" , Alan Stern Date: Tue, 14 Jan 2020 15:41:39 +0100 In-Reply-To: References: <20200113033035.50875-1-ejh@nvidia.com> <1578908680.2590.10.camel@suse.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-usb-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org Am Dienstag, den 14.01.2020, 03:28 +0000 schrieb EJ Hsu: > Oliver Neukum wrote: > > > Am Sonntag, den 12.01.2020, 19:30 -0800 schrieb EJ Hsu: > > > > Isn't that the bug? A command to a detached device should fail. > > Could you please elaborate? This issue would not be limited to uas. > > > > In the case I mentioned, the hub thread of external hub running > uas_probe() will get stuck waiting for the completion of scsi scan. > > The scsi scan will try to probe a single LUN using a SCSI INQUIRY. > If the external hub has been unplugged before LUN probe, the device > state of uas device will be set to USB_STATE_NOTATTACHED by the > root hub thread. So, all the following calls to usb_submit_urb() in > uas driver will return -NODEV, and accordingly uas_queuecommand_lck() > will return SCSI_MLQUEUE_DEVICE_BUSY to scsi_request_fn(). And that looks like the root cause. The queue isn't busy. It is dead. > scsi_request_fn() then puts this scsi command back into request queue. > Because this scsi device is just created and during LUN probe process, > this scsi command is the only one in the request queue. So, it will be picked > up soon and dispatched to uas driver again. This cycle will continue until > uas_disconnect() is called and its "resetting" flag is set. However, the > hub thread of external hub still got stuck waiting for the completion of > this scsi command, and may not be able to run uas_disconnect(). > A deadlock happened. I see. But we are working around insufficient error reporting in the SCSI midlayer. > > > +static void uas_scan_work(struct work_struct *work) { > > > + struct uas_dev_info *devinfo = > > > + container_of(work, struct uas_dev_info, scan_work); > > > + struct Scsi_Host *shost = usb_get_intfdata(devinfo->intf); > > > + > > > + dev_dbg(&devinfo->intf->dev, "starting scan\n"); > > > + scsi_scan_host(shost); > > > + dev_dbg(&devinfo->intf->dev, "scan complete\n"); > > > + > > > + usb_autopm_put_interface(devinfo->intf); > > > > scsi_scan_host() does runtime PM on the SCSI level. There is no need for us to > > duplicate that. > > > > In my opinion, if scsi_scan_host() will be run asynchronously, this interface > needs to be guarded against runtime PM between uas_probe() & uas_scan_work(). Yes it does. But it has a child, the SCSI host, which has an elevated count. It is already guarded. Regards Oliver