linux-kernel - Re: [PATCH net-next] hv_netvsc: fix a race between netvsc_send() and netvsc_init

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Message-Id: <20161020.140300.122827393647670926.davem@davemloft.net>
Date:   Thu, 20 Oct 2016 14:03:00 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     vkuznets@...hat.com
Cc:     sthemmin@...rosoft.com, netdev@...r.kernel.org,
        devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
        kys@...rosoft.com, haiyangz@...rosoft.com
Subject: Re: [PATCH net-next] hv_netvsc: fix a race between netvsc_send()
 and netvsc_init_buf()

From: Vitaly Kuznetsov <vkuznets@...hat.com>
Date: Thu, 20 Oct 2016 10:51:04 +0200

> Stephen Hemminger <sthemmin@...rosoft.com> writes:
> 
>> Do we need ACCESS_ONCE() here to avoid check/use issues?
>>
> 
> I think we don't: this is the only place in the function where we read
> the variable so we'll get normal read. We're not trying to syncronize
> with netvsc_init_buf() as that would require locking, if we read stale
> NULL value after it was already updated on a different CPU we're fine,
> we'll just return -EAGAIN.

The concern is if we race with netvsc_destroy_buf() and this pointer
becomes NULL after the test you are adding.