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Message-ID: <1952182.1698853516@warthog.procyon.org.uk>
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
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