linux-kernel - Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Message-ID: <20160217110520.GN6375@twins.programming.kicks-ass.net>
Date:	Wed, 17 Feb 2016 12:05:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Waiman Long <Waiman.Long@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

On Wed, Feb 17, 2016 at 12:00:40PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 17, 2016 at 08:53:18PM +1100, Dave Chinner wrote:
> > > +/**
> > > + * for_all_percpu_list_entries - iterate over all the per-cpu list with locking

But the locking is 'pointless'. You only lock one per-cpu sublist at a
time, therefore the list can be modified concurrently anyhow.

So why not use RCU for the list iteration and avoid potentially large
lock hold times?

> > > + * @pos:	the type * to use as a loop cursor for the current .
> > > + * @next:	an internal type * variable pointing to the next entry
> > > + * @pchead:	an internal struct list * of percpu list head
> > > + * @pclock:	an internal variable for the current per-cpu spinlock
> > > + * @head:	the head of the per-cpu list
> > > + * @member:	the name of the per-cpu list within the struct
> > > + */