linux-kernel - Re: [PATCH net-next 3/4] hv_netvsc: reset net_device_ctx->nvdev with rcu_assign

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Message-ID: <20171031174533.6877e1a5@shemminger-XPS-13-9360>
Date:   Tue, 31 Oct 2017 17:45:33 +0100
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        netdev@...r.kernel.org, Haiyang Zhang <haiyangz@...rosoft.com>,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org
Subject: Re: [PATCH net-next 3/4] hv_netvsc: reset net_device_ctx->nvdev
 with rcu_assign_pointer()

On Tue, 31 Oct 2017 07:09:58 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Tue, 2017-10-31 at 14:42 +0100, Vitaly Kuznetsov wrote:
> > RCU_INIT_POINTER() is not suitable here as it doesn't give us ordering
> > guarantees (see the comment in rcupdate.h). This is also not a hotpath.
> > 
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> > ---
> >  drivers/net/hyperv/netvsc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
> > index bfc79698b8f4..12efb3e34775 100644
> > --- a/drivers/net/hyperv/netvsc.c
> > +++ b/drivers/net/hyperv/netvsc.c
> > @@ -560,7 +560,7 @@ void netvsc_device_remove(struct hv_device *device)
> >  
> >  	netvsc_revoke_buf(device, net_device);
> >  
> > -	RCU_INIT_POINTER(net_device_ctx->nvdev, NULL);
> > +	rcu_assign_pointer(net_device_ctx->nvdev, NULL);  
> 
> I see no point for this patch.
> 
> Setting a NULL pointer needs no barrier at all.

Agreed with Eric.