From: Oleg Nesterov <oleg@redhat.com>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org, bristot@redhat.com,
peterz@infradead.org, paulmck@kernel.org, rcu@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@linux.ibm.com>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] Remove GP_REPLAY state from rcu_sync
Date: Fri, 4 Oct 2019 17:41:03 +0200 [thread overview]
Message-ID: <20191004154102.GA20945@redhat.com> (raw)
In-Reply-To: <20191004145741.118292-1-joel@joelfernandes.org>
On 10/04, Joel Fernandes (Google) wrote:
>
> But this is not always true if you consider the following events:
I'm afraid I missed your point, but...
> ---------------------->
> GP num 111111 22222222222222222222222222222222233333333
> GP state i e p x r rx i
> CPU0 : rse rsx
> CPU1 : rse rsx
> CPU2 : rse rsx
>
> Here, we had 3 grace periods that elapsed, 1 for the rcu_sync_enter(),
> and 2 for the rcu_sync_exit(s).
But this is fine?
We only need to ensure that we have a full GP pass between the "last"
rcu_sync_exit() and GP_XXX -> GP_IDLE transition.
> However, we had 3 rcu_sync_exit()s, not 2. In other words, the
> rcu_sync_exit() got batched.
>
> So my point here is, rcu_sync_exit() does not really always cause a new
> GP to happen
See above, it should not.
> Then what is the point of the GP_REPLAY state at all if it does not
> always wait for a new GP?
Again, I don't understand... GP_REPLAY ensures that we will have a full GP
before rcu_sync_func() sets GP_IDLE, note that it does another "recursive"
call_rcu() if it sees GP_REPLAY.
> Taking a step back, why did we intend to have
> to wait for a new GP if another rcu_sync_exit() comes while one is still
> in progress?
To ensure that if another CPU sees rcu_sync_is_idle() (GP_IDLE) after you
do rcu_sync_exit(), then it must also see all memory changes you did before
rcu_sync_exit().
Oleg.
next prev parent reply other threads:[~2019-10-04 15:41 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 14:57 [PATCH] Remove GP_REPLAY state from rcu_sync Joel Fernandes (Google)
2019-10-04 14:59 ` Joel Fernandes
2019-10-04 15:41 ` Oleg Nesterov [this message]
2019-10-04 16:37 ` Joel Fernandes
2019-10-07 14:09 ` Oleg Nesterov
2020-01-16 21:57 ` Joel Fernandes
2019-10-04 19:25 ` kbuild test robot
2019-10-04 22:03 ` kbuild test robot
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