linux-kernel - Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather than read_seqbegin_or

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Message-ID: <20231101224855.GJ1957730@ZenIV>
Date:   Wed, 1 Nov 2023 22:48:55 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        linux-afs@...ts.infradead.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather
 than read_seqbegin_or_lock()

On Wed, Nov 01, 2023 at 10:52:15PM +0100, Oleg Nesterov wrote:

> > Why would you want to force that "switch to locked on the second pass" policy
> > on every possible caller?
> 
> Because this is what (I think) read_seqbegin_or_lock() is supposed to do.
> It should take the lock for writing if the lockless access failed. At least
> according to the documentation.

Not really - it's literally seqbegin or lock, depending upon what the caller
tells it...  IMO the mistake in docs is the insistence on using do-while
loop for its users.

Take a look at d_walk() and try to shoehorn that into your variant.  Especially
the D_WALK_NORETRY handling...