linux-kernel - RE: [EXTERNAL] Re: [PATCH net v2 1/4] octeon_ep: fix race conditions in ndo_get

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Message-ID:
 <CO1PR18MB472973A60723E9417FE1BB87C7042@CO1PR18MB4729.namprd18.prod.outlook.com>
Date: Tue, 17 Dec 2024 17:49:13 +0000
From: Shinas Rasheed <srasheed@...vell.com>
To: Larysa Zaremba <larysa.zaremba@...el.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Haseeb Gani
	<hgani@...vell.com>, Sathesh B Edara <sedara@...vell.com>,
        Vimlesh Kumar
	<vimleshk@...vell.com>,
        "thaller@...hat.com" <thaller@...hat.com>,
        "wizhao@...hat.com" <wizhao@...hat.com>,
        "kheib@...hat.com"
	<kheib@...hat.com>,
        "konguyen@...hat.com" <konguyen@...hat.com>,
        "horms@...nel.org" <horms@...nel.org>,
        "einstein.xue@...axg.com"
	<einstein.xue@...axg.com>,
        Veerasenareddy Burru <vburru@...vell.com>,
        Andrew
 Lunn <andrew+netdev@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni
	<pabeni@...hat.com>,
        Abhijit Ayarekar <aayarekar@...vell.com>,
        Satananda
 Burla <sburla@...vell.com>
Subject: RE: [EXTERNAL] Re: [PATCH net v2 1/4] octeon_ep: fix race conditions
 in ndo_get_stats64



> > > On Mon, Dec 16, 2024 at 03:30:12PM +0100, Larysa Zaremba wrote:
> > > > On Sun, Dec 15, 2024 at 11:58:39PM -0800, Shinas Rasheed wrote:
> > > > > ndo_get_stats64() can race with ndo_stop(), which frees input and
> > > > > output queue resources. Call synchronize_net() to avoid such races.
> > > > >
> > > > > Fixes: 6a610a46bad1 ("octeon_ep: add support for ndo ops")
> > > > > Signed-off-by: Shinas Rasheed <srasheed@...vell.com>
> > > > > ---
> > > > > V2:
> > > > >   - Changed sync mechanism to fix race conditions from using an
> atomic
> > > > >     set_bit ops to a much simpler synchronize_net()
> > > > >
> > > > >  drivers/net/ethernet/marvell/octeon_ep/octep_main.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> > > b/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> > > > > index 549436efc204..941bbaaa67b5 100644
> > > > > --- a/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> > > > > +++ b/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> > > > > @@ -757,6 +757,7 @@ static int octep_stop(struct net_device
> *netdev)
> > > > >  {
> > > > >  	struct octep_device *oct = netdev_priv(netdev);
> > > > >
> > > > > +	synchronize_net();
> > > >
> > > > You should have elaborated on the fact that this synchronize_net() is for
> > > > __LINK_STATE_START flag in the commit message, this is not obvious.
> Also,
> > > is
> > > > octep_get_stats64() called from RCU-safe context?
> > > >
> > >
> > > Now I see that in case !netif_running(), you do not bail out of
> > > octep_get_stats64() fully (or at all after the second patch). So, could you
> > > explain, how are you utilizing RCU here?
> > >
> >
> > The understanding is that octep_get_stats64() (.ndo_get_stats64() in turn) is
> called from RCU safe contexts, and
> > that the netdev op is never called after the ndo_stop().
> 
> As I now see, in net/core/net-sysfs.c, yes there is an rcu read lock around the
> thing, but there are a lot more callers and for example veth_get_stats64()
> explicitly calls rcu_read_lock().
> 
> Also, even with RCU-protected section, I am not sure prevents the
> octep_get_stats64() to be called after synchronize_net() finishes. Again, the
> callers seem too diverse to definitely say that we can rely on built-in flags
> for this to not happen :/

Usually, the understanding is that ndo_get_stats won't be called by the network stack after the interface is put down. As long as that is the case, I don't think we should keep adding checks until there is a strong reason to do so. What do you think?

> > Thanks for the comments