linux-kernel - Re: [PATCH v3 11/13] xen/pvcalls: implement poll command

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Date:   Tue, 12 Sep 2017 10:14:00 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v3 11/13] xen/pvcalls: implement poll command


>>> +
>>> +unsigned int pvcalls_front_poll(struct file *file, struct socket *sock,
>>> +			       poll_table *wait)
>>> +{
>>> +	struct pvcalls_bedata *bedata;
>>> +	struct sock_mapping *map;
>>> +
>>> +	if (!pvcalls_front_dev)
>>> +		return POLLNVAL;
>>> +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
>>> +
>>> +	map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
>> I just noticed this --- why is it READ_ONCE? Are you concerned that
>> sk_send_head may change?
> No, but I wanted to avoid partial reads. A caller could call
> pvcalls_front_accept and pvcalls_front_poll on newsock almost at the
> same time (it is probably not the correct way to use the API), I wanted
> to make sure that "map" is either read correctly, or not read at all.

How can you have a partial read on a pointer?

-boris