linux-kernel - Re: [PATCH 1/5] rcusync: introduce struct rcu_sync

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Date:	Fri, 4 Oct 2013 21:22:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] rcusync: introduce struct rcu_sync_ops

On 10/04, Linus Torvalds wrote:
>
> On Fri, Oct 4, 2013 at 11:46 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> > Add the new struct rcu_sync_ops which holds sync/call methods, and
> > turn the function pointers in rcu_sync_struct into the single pointer
> > to struct rcu_sync_ops.
> >
> > +struct rcu_sync_ops {
> > +       void (*sync)(void);
> > +       void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> > +};
> > +
> >  struct rcu_sync_struct {
> >         int                     gp_state;
> >         int                     gp_count;
> > @@ -12,43 +17,9 @@ struct rcu_sync_struct {
> >         int                     cb_state;
> >         struct rcu_head         cb_head;
> >
> > -       void (*sync)(void);
> > -       void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> > +       struct rcu_sync_ops     *ops;
>
> Ugh.
>
> This interface pretty much guarantees that a compiler can never do
> anything clever, like know that "hey, you used a static initializer on
> this thing, and the fields are const, so now know statically what the
> functions are, and I can just turn the indirect jumps into direct
> jumps".

But we do not care? rcu_sync_struct->ops is only used by the writer
(slow path). In this case the simpler the better, I think.

>  - instead, use a "static const" type descriptor for each type (it
> approaches being your "rcu_sync_ops" structure). Pass this in as an
> argument to all the functions (use a #define per type or something, so
> that users don't need to do this by hand)
>
>  - now every single user passes in that type descriptor.
>
>  - together with using a few inline functions, suddenly the "indirect"
> jumps through this type descriptor end up actually being nice direct
> compile-time constants: iow, they get turned into direct jumps.

Hmm. Can't understand, sorry... Could you spell??

I assume you do not suggest to pass the "type" to, say, rcu_sync_enter?

Oleg.

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