linux-kernel - Re: [PATCH v4 12/18] xen/pvcalls: implement poll command

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Date:   Wed, 21 Jun 2017 14:49:20 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v4 12/18] xen/pvcalls: implement poll command

On Wed, 21 Jun 2017, Boris Ostrovsky wrote:
> >>> +
> >>> +	mappass->reqcopy = *req;
> >>> +	icsk = inet_csk(mappass->sock->sk);
> >>> +	queue = &icsk->icsk_accept_queue;
> >>> +	spin_lock(&queue->rskq_lock);
> >>> +	data = queue->rskq_accept_head != NULL;
> >>> +	spin_unlock(&queue->rskq_lock);
> >> What is the purpose of the queue lock here?
> > It is only there to protect accesses to rskq_accept_head. Functions that
> > change rskq_accept_head take this lock, see for example
> > net/ipv4/inet_connection_sock.c:inet_csk_reqsk_queue_add. I'll add an
> > in-code comment.
> 
> I am not sure I follow. You are not changing rskq_accept_head, you are
> simply reading it under the lock. It may be set by others to NULL as
> soon as you drop the lock, at which point 'data' test below will be
> obsolete.
> 
> In inet_csk_reqsk_queue_add() it is read and then, based on read result,
> is written with a value so a lock is indeed need there.

I think you are right. The only thing is that without the lock we might
read a transitory value as the rskq_accept_head reads/writes are not
guaranteed to be atomic. However, I don't think we care about it, since
this is just a != NULL test and, as you wrote, the result could be
obsolete immediately after. I'll drop the lock.



> >
> >
> >>> +	if (data) {
> >>> +		mappass->reqcopy.cmd = 0;
> >>> +		ret = 0;
> >>> +		goto out;
> >>> +	}
> >>> +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> >>> +
> >>> +	/* Tell the caller we don't need to send back a notification yet */
> >>> +	return -1;
>