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

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Message-ID: <20231027100047.GA30884@redhat.com>
Date: Fri, 27 Oct 2023 12:00:47 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: 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>
Cc: 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 10/27, Oleg Nesterov wrote:
>
> read_seqbegin_or_lock() makes no sense unless you make "seq" odd
> after the lockless access failed. See thread_group_cputime() as
> an example, note that it does nextseq = 1 for the 2nd round.

See also

	[PATCH 1/2] seqlock: fix the wrong read_seqbegin_or_lock/need_seqretry documentation
	https://lore.kernel.org/all/20231024120808.GA15382@redhat.com/

> So this code can use read_seqbegin() without changing the current
> behaviour.

I am trying to remove the misuse of read_seqbegin_or_lock(),
then I am going to turn need_seqretry() into

	static inline int need_seqretry(seqlock_t *lock, int *seq)
	{
		int ret = !(*seq & 1) && read_seqretry(lock, *seq);

		if (ret)
			*seq = 1; /* make this counter odd */

		return ret;
	}

Oleg.