From: Oliver Neukum <oneukum@suse.com>
To: EJ Hsu <ejh@nvidia.com>
Cc: "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [PATCH] usb: uas: fix a plug & unplug racing
Date: Tue, 14 Jan 2020 15:41:39 +0100 [thread overview]
Message-ID: <1579012899.15925.7.camel@suse.com> (raw)
In-Reply-To: <MN2PR12MB4126334FBC97B5AC4CC442AECF340@MN2PR12MB4126.namprd12.prod.outlook.com>
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
next prev parent reply other threads:[~2020-01-14 14:41 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-13 3:30 [PATCH] usb: uas: fix a plug & unplug racing EJ Hsu
2020-01-13 9:44 ` Oliver Neukum
2020-01-13 15:21 ` Alan Stern
2020-01-14 3:28 ` EJ Hsu
2020-01-14 14:41 ` Oliver Neukum [this message]
2020-01-14 15:04 ` Alan Stern
2020-01-15 9:31 ` Oliver Neukum
2020-01-15 15:17 ` Alan Stern
2020-01-15 15:54 ` EJ Hsu
2020-01-20 9:38 ` Oliver Neukum
2020-01-21 11:29 ` EJ Hsu
2020-01-22 9:52 ` EJ Hsu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1579012899.15925.7.camel@suse.com \
--to=oneukum@suse.com \
--cc=ejh@nvidia.com \
--cc=linux-usb@vger.kernel.org \
--cc=stern@rowland.harvard.edu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).