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

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Date: Wed, 01 Nov 2023 15:45:16 +0000
From: David Howells <dhowells@...hat.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: dhowells@...hat.com, Marc Dionne <marc.dionne@...istor.com>,
    Alexander Viro <viro@...iv.linux.org.uk>,
    "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()

Oleg Nesterov <oleg@...hat.com> wrote:

> read_seqbegin_or_lock() makes no sense unless you make "seq" odd
> after the lockless access failed.

I think you're wrong.

write_seqlock() turns it odd.  For instance, if the read lock is taken first:

	sequence seq	CPU 1				CPU 2
	======= =======	===============================	===============
	0
	0	0	seq = 0 // MUST BE EVEN ACCORDING TO DOC
	0	0	read_seqbegin_or_lock() [lockless]
			...
	1	0					write_seqlock()
	1	0	need_seqretry() [seq=even; sequence!=seq: retry]
	1	1	read_seqbegin_or_lock() [exclusive]
			-->spin_lock(lock);
	2	1					write_sequnlock()
			<--locked
			...
	2	1	need_seqretry()

However, if the write lock is taken first:

	sequence seq	CPU 1				CPU 2
	======= =======	===============================	===============
	0
	1						write_seqlock()
	1	0	seq = 0 // MUST BE EVEN ACCORDING TO DOC
	1	0	read_seqbegin_or_lock() [lockless]
	1	0	    __read_seqcount_begin()
				while (lock.sequence is odd)
				    cpu_relax();
	2	0					write_sequnlock()
	2	2		[loop end]
			...
	2	2	need_seqretry() [seq=even; sequence==seq; done]

Note that it spins in __read_seqcount_begin() until we get an even seq,
indicating that no write is currently in progress - at which point we can
perform a lockless pass.

> See thread_group_cputime() as an example, note that it does nextseq = 1 for
> the 2nd round.

That's not especially convincing.

David