linux-kernel - Re: [PATCH v2 18/27] rcu: Rename rcu_dynticks_in_eqs_since() into rcu_watching_changed

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Date: Tue, 7 May 2024 15:48:18 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 18/27] rcu: Rename rcu_dynticks_in_eqs_since() into
 rcu_watching_changed_since()

Le Tue, Apr 30, 2024 at 11:17:22AM +0200, Valentin Schneider a écrit :
> The context_tracking.state RCU_DYNTICKS subvariable has been renamed to
> RCU_WATCHING, the dynticks prefix can go.
> 
> Furthermore, the "in_eqs_since" part confuses me, as IIUC this only checks
> for a change in watching/eqs state, not that RCU transitionned *into* a
> EQS after the snapshot was taken.
> 
> e.g. if
>   snap = 0b1000 (EQS)
> and the following rcu_watching_snap(CPU) is:
> 	 0b1100 (watching)
> then
>   rcu_watching_in_eqs_since(rdp, snap) -> true
> 
> but because RCU was already in EQS at the time of the
> snap - it hasn't entered EQS "since" the snap was taken.
> 
> Update the name to reflect that we're only looking at watching/EQS
> transitions, not specifically transitions into EQS.

Indeed in practice the function only checks a change. But semantically it really
checks a trip to eqs because this function is only ever called after a failing
call to rcu_dynticks_in_eqs().

So not sure about that one rename. Paul?