netdev - Re: Missing a write memory barrier in tls

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Date: Tue, 7 Nov 2023 18:53:24 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Sabrina Dubroca <sd@...asysnail.net>
Cc: "Dae R. Jeong" <threeearcat@...il.com>, borisp@...dia.com,
 john.fastabend@...il.com, davem@...emloft.net, edumazet@...gle.com,
 pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 ywchoi@...ys.kaist.ac.kr
Subject: Re: Missing a write memory barrier in tls_init()

On Tue, 7 Nov 2023 23:45:46 +0100 Sabrina Dubroca wrote:
> Wouldn't it be enough to just move the rcu_assign_pointer after ctx is
> fully initialized, ie just before update_sk_prot? also clearer wrt
> RCU.

I'm not sure, IIUC rcu_assign_pointer() is equivalent to
WRITE_ONCE() on any sane architecture, it depends on address
dependencies to provide ordering. Since here we care about
ctx->sk_prot being updated, when changes to sk->sk_prot
are visible there is no super-obvious address dependency.

There may be one. But to me at least it isn't an obvious
"RCU used right will handle this" case.

> (and maybe get rid of tls_ctx_create and move all that into tls_init,
> it's not much and we don't even set ctx->{tx,rx}_conf in there)