netdev - Re: [PATCH net-next] net/tls: Use RCU API to access tls

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Message-ID: <380eb27278e581012524cdc16f99e1872cee9be0.camel@nvidia.com>
Date:   Tue, 2 Aug 2022 12:03:32 +0000
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     "kuba@...nel.org" <kuba@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...dia.com>,
        Gal Pressman <gal@...dia.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Boris Pismenny <borisp@...dia.com>
Subject: Re: [PATCH net-next] net/tls: Use RCU API to access tls_ctx->netdev

On Mon, 2022-08-01 at 12:42 -0700, Jakub Kicinski wrote:
> On Mon, 1 Aug 2022 11:00:53 +0300 Maxim Mikityanskiy wrote:
> > @@ -1329,7 +1345,11 @@ static int tls_device_down(struct net_device *netdev)
> >  
> >  	spin_lock_irqsave(&tls_device_lock, flags);
> >  	list_for_each_entry_safe(ctx, tmp, &tls_device_list, list) {
> > -		if (ctx->netdev != netdev ||
> > +		struct net_device *ctx_netdev =
> > +			rcu_dereference_protected(ctx->netdev,
> > +						  lockdep_is_held(&device_offload_lock));
> > +
> > +		if (ctx_netdev != netdev ||
> >  		    !refcount_inc_not_zero(&ctx->refcount))
> >  			continue;
> 
> For cases like this where we don't actually hold onto the object, just
> take a peek at the address of it we can save a handful of LoC by using
> rcu_access_pointer(). 

The documentation of rcu_access_pointer says it shouldn't be used on
the update side, because we lose lockdep protection:

--cut--

Although rcu_access_pointer() may also be used in cases
where update-side locks prevent the value of the pointer from changing,
you should instead use rcu_dereference_protected() for this use case.

--cut--

Though, I can change the read side to use rcu_access_pointer, wherever
the pointer is not dereferenced. But it won't save lines of code :)