linux-kernel - Re: [PATCH] vhost: Take a reference on the task that is reference in struct vhost

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Message-ID: <20250921173934-mutt-send-email-mst@kernel.org>
Date: Sun, 21 Sep 2025 17:40:01 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: Take a reference on the task that is reference in
 struct vhost_task.

On Sun, Sep 21, 2025 at 04:56:20PM -0400, Michael S. Tsirkin wrote:
> Subject: that is reference -> that is referenced

to note i fixed it for now. just dropped "that is referenced"
completely. shorter.

> On Thu, Sep 18, 2025 at 08:11:44PM +0200, Sebastian Andrzej Siewior wrote:
> > vhost_task_create() creates a task and keeps a reference to its
> > task_struct. That task may exit early via a signal and its task_struct
> > will be released.
> > A pending vhost_task_wake() will then attempt to wake the task and
> > access a task_struct which is no longer there.
> > 
> > Acquire a reference on the task_struct while creating the thread and
> > release the reference while the struct vhost_task itself is removed.
> > If the task exits early due to a signal, then the vhost_task_wake() will
> > still access a valid task_struct. The wake is safe and will be skipped
> > in this case.
> > 
> > Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
> > Reported-by: Sean Christopherson <seanjc@...gle.com>
> > Closes: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com/
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  kernel/vhost_task.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> > index bc738fa90c1d6..27107dcc1cbfe 100644
> > --- a/kernel/vhost_task.c
> > +++ b/kernel/vhost_task.c
> > @@ -100,6 +100,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
> >  	 * freeing it below.
> >  	 */
> >  	wait_for_completion(&vtsk->exited);
> > +	put_task_struct(vtsk->task);
> >  	kfree(vtsk);
> >  }
> >  EXPORT_SYMBOL_GPL(vhost_task_stop);
> > @@ -148,7 +149,7 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
> >  		return ERR_CAST(tsk);
> >  	}
> >  
> > -	vtsk->task = tsk;
> > +	vtsk->task = get_task_struct(tsk);
> >  	return vtsk;
> >  }
> >  EXPORT_SYMBOL_GPL(vhost_task_create);
> > -- 
> > 2.51.0