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

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Date:   Fri, 5 Aug 2022 10:59:42 +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>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        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 Thu, 2022-08-04 at 09:18 -0700, Jakub Kicinski wrote:
> On Thu, 4 Aug 2022 08:08:37 +0000 Maxim Mikityanskiy wrote:
> > 2. ctx->refcount goes down to 0, no one can access ctx->netdev anymore,
> > we tear down ctx and need to check whether ctx->netdev is NULL.
> > 
> >  if (!refcount_dec_and_test(&ctx->refcount))
> >          return;
> >  netdev = rcu_dereference_protected(ctx->netdev,
> >                                     !refcount_read(&ctx->refcount));
> >  if (netdev)
> >          queue_work(...);
> > 
> > It's somewhat similar to the "structure is beyond being updated" case,
> > but it's ensured by refcount, not by RCU (i.e. you example assigned
> > my_rcu_pointer = NULL and called synchronize_rcu() to ensure no one
> > touches it, and I ensure that we are the only user of ctx by dropping
> > refcount to zero).
> > 
> > So, hoping that my understanding of your explanation is correct, both
> > cases can use any of rcu_access_pointer or rcu_dereference_protected.
> > Is there some rule of thumb which one to pick in such case?
> 
> IMHO, rcu_dereference_protected() documents why it's safe to
> dereference the pointer outside of the rcu read section. 
> 
> We are only documenting the writer side locking. The fact that there
> is a RCU pointer involved is coincidental - I think we could 
> as well be checking the TLS_RX_DEV_DEGRADED bit here.

Yes, checking TLS_RX_DEV_DEGRADED would be equivalent in the current
implementation. But I don't want to give this flag extra
responsibilities (currently it's for RX resync only) if we can check
the netdev pointer, it should be more flexible in long term.

> We can add asserts or comments to document the writer side locking.
> Piggy backing on RCU seems coincidental. But again, I'm fine either 
> way. I'm just saying this because I really doubt there is a rule of
> thumb for this level of detail. It's most likely your call.

OK, I'd keep rcu_dereference_protected with its asserts, rather than
reinvent my own asserts.