From: Oleg Nesterov <oleg@redhat.com>
To: Tobias Klauser <tklauser@distanz.ch>,
Matt Mooney <mfm@muteddisk.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Subject: ping: drivers/staging/usbip/ abuses task_is_dead/exit_state
Date: Tue, 6 Mar 2012 18:39:25 +0100 [thread overview]
Message-ID: <20120306173925.GA17551@redhat.com> (raw)
In-Reply-To: <20110920183810.GA25159@suse.de>
On 09/20, Greg KH wrote:
>
> On Tue, Sep 20, 2011 at 05:14:10PM +0200, Oleg Nesterov wrote:
> > (add more cc's)
> >
> > On 09/20, Oleg Nesterov wrote:
> > >
> > > Unfortunately, we can't kill task_is_dead() right now, it has already
> > > found the users in drivers/staging/, and I bet the usage is wrong.
> >
> > It is used by drivers/staging/usbip/
> >
> > For what? The code:
> >
> > if (vdev->ud.tcp_rx && !task_is_dead(vdev->ud.tcp_rx))
> > kthread_stop(vdev->ud.tcp_rx);
> >
> > And how task_is_dead() can help? This helper is really "special", it
> > shouldn't be used anyway. But why do we check ->exit_state? Without
> > tasklist the check is racy anyway, the task can exit right after the
> > check.
> >
> > And. It is safe to use kthread_stop(t) even if t has already exited.
> >
> > OK, this was added by 8547d4cc2b616e4f1dafebe2c673fc986422b506
> > "Staging: usbip: vhci-hcd: Do not kill already dead RX/TX kthread"
> >
> > When unbinding a device on the host which was still attached on the
> > client, I got a NULL pointer dereference on the client.
> >
> > Where?
> >
> > This turned out
> > to be due to kthread_stop() being called on an already dead kthread.
> >
> > This should work.
> >
> > I'm afraid this can only fix the symptom. Probably, the problem is that
> > we do not have the reference and thus even task_is_dead(t) is not safe.
> >
> > This kthread was created by kthread_run(). If it exits, nothing protects
> > this task_struct.
> >
> > In any case, please do not use ->exit_state. It should not be used outside
> > of exit.c/etc paths, "exit_state != 0" means "exit_notify() was called".
>
> Patches to fix this up in this driver are always gladly appreciated :)
OK, since nobody cares, probably I should make the patch even if I don't
understand this code at all and can't test the change.
But, Tobias, may be you can explain what this task_is_dead() check was
supposed to do?
Oleg.
next prev parent reply other threads:[~2012-03-06 18:26 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-19 21:45 [PATCH] user namespace: make signal.c respect user namespaces Serge E. Hallyn
2011-09-19 21:47 ` [PATCH] user namespace: usb: make usb urbs user namespace aware Serge E. Hallyn
2011-09-20 13:17 ` Oleg Nesterov
2011-09-20 13:33 ` Serge E. Hallyn
2011-09-21 5:01 ` [PATCH] user namespace: usb: make usb urbs user namespace aware (v2) Serge E. Hallyn
2011-09-21 18:31 ` Oleg Nesterov
2011-09-21 19:12 ` Serge E. Hallyn
2011-09-21 19:18 ` Greg KH
2011-09-23 1:27 ` [PATCH resend] " Serge E. Hallyn
2011-09-23 15:48 ` Alan Stern
2011-09-23 16:06 ` Serge E. Hallyn
2011-09-23 16:21 ` Alan Stern
2011-09-23 17:22 ` Serge E. Hallyn
2011-09-23 18:35 ` Alan Stern
2011-09-20 12:22 ` [PATCH] user namespace: make signal.c respect user namespaces Oleg Nesterov
2011-09-20 12:44 ` Serge E. Hallyn
2011-09-20 13:41 ` Oleg Nesterov
2011-09-20 14:39 ` [PATCH 0/2] (Was: user namespace: make signal.c respect user namespaces) Oleg Nesterov
2011-09-20 14:39 ` [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check Oleg Nesterov
2011-09-20 15:14 ` drivers/staging/usbip/ abuses task_is_dead/exit_state Oleg Nesterov
2011-09-20 18:38 ` Greg KH
2012-03-06 17:39 ` Oleg Nesterov [this message]
2012-03-06 19:30 ` ping: " Tobias Klauser
2012-03-08 18:57 ` Oleg Nesterov
2012-03-13 11:45 ` Tobias Klauser
2012-03-13 18:07 ` [PATCH] staging: usbip: fix the usage of kthread_stop() Oleg Nesterov
2012-04-01 23:17 ` Oleg Nesterov
2012-04-02 8:11 ` Tobias Klauser
2011-09-20 15:28 ` [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check Paul E. McKenney
2011-09-20 15:40 ` Oleg Nesterov
2011-09-20 15:48 ` Paul E. McKenney
2011-09-20 14:39 ` [PATCH 2/2] creds: __task_cred(current) doesn't need rcu_read_lock_held() Oleg Nesterov
2011-09-20 15:07 ` Serge Hallyn
2011-09-20 15:35 ` Oleg Nesterov
2011-09-20 16:19 ` David Howells
2011-09-20 16:38 ` Oleg Nesterov
2011-09-20 16:50 ` David Howells
2011-09-20 17:13 ` Oleg Nesterov
2011-09-20 16:27 ` [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check David Howells
2011-09-20 15:39 ` [PATCH] user namespace: make signal.c respect user namespaces Serge Hallyn
2011-09-20 16:24 ` Oleg Nesterov
2011-09-20 16:45 ` Serge E. Hallyn
2011-09-20 18:17 ` Oleg Nesterov
2011-09-21 5:00 ` [PATCH] user namespace: make signal.c respect user namespaces (v2) Serge E. Hallyn
2011-09-20 17:48 ` [PATCH] user namespace: make signal.c respect user namespaces Oleg Nesterov
2011-09-20 18:53 ` Serge E. Hallyn
2011-09-21 17:53 ` Oleg Nesterov
2011-09-22 15:23 ` Serge Hallyn
2011-09-23 16:31 ` Serge E. Hallyn
2011-09-23 17:36 ` Oleg Nesterov
2011-09-23 21:20 ` Serge E. Hallyn
2011-09-24 16:37 ` Oleg Nesterov
2011-09-25 20:17 ` Serge E. Hallyn
2011-09-26 16:06 ` Oleg Nesterov
2011-09-27 14:28 ` Serge Hallyn
2011-09-27 14:38 ` Oleg Nesterov
2011-09-27 15:27 ` Serge Hallyn
2011-09-27 17:12 ` Oleg Nesterov
2011-10-04 17:42 ` Serge E. Hallyn
2011-10-09 19:00 ` Oleg Nesterov
2011-10-11 13:08 ` Serge E. Hallyn
2011-10-08 20:02 ` Serge E. Hallyn
2011-10-09 19:03 ` Oleg Nesterov
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=20120306173925.GA17551@redhat.com \
--to=oleg@redhat.com \
--cc=gregkh@linuxfoundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mfm@muteddisk.com \
--cc=tklauser@distanz.ch \
/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).