linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

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Message-ID: <20100113234244.GA15897@Krystal>
Date:	Wed, 13 Jan 2010 18:42:44 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Nicholas Miell <nmiell@...cast.net>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier (v5)

* Nicholas Miell (nmiell@...cast.net) wrote:
> On Wed, 2010-01-13 at 11:42 -0800, David Daney wrote:
> > Mathieu Desnoyers wrote:
> > > * Nicholas Miell (nmiell@...cast.net) wrote:
> > >> On Wed, 2010-01-13 at 13:24 -0500, Mathieu Desnoyers wrote:
> > >>> * Nicholas Miell (nmiell@...cast.net) wrote:
> > >>>
> > >>>> The whole point of compat and incompat flags is that it allows new
> > >>>> applications to run on old kernels and either work or fail as
> > >>>> appropriate, depending on whether the new features they're using must be
> > >>>> implemented or can be silently ignored.
> > >>> I see. Thanks for the explanation. Then the expedited flag should
> > >>> clearly be part of the mandatory flags.
> > >>>
> > >>> Can you point me to other system calls that are doing this ?
> > >>>
> > >>> Thanks,
> > >>>
> > >>> Mathieu
> > >> Not off the top of my head, but I did steal the idea from the ext2/3/4
> > >> disk format.
> > > 
> > > Sounds a bit over-engineered to me for system calls, but who knows if we
> > > eventually have to extend sys_membarrier(). This involves that, right
> > > now, I'd have to add a header to include/linux to define these flags.
> > > Also, "int expedited" is a bit clearer, but less flexible, than "int
> > > flags". Anyone else have comments about this ?
> > > 
> > 
> > It doesn't bother me that you have to do extra work to add the flag 
> > definitions to a header file. :-)
> > 

Work is not a problem. I just wanted to make sure I would not be going
in circles. :)

> > As I understand it, the proposal is to have the option to extend the ABI 
> > based on as yet undefined flag bits.  This doesn't seem like a bad thing.
> > 
> > The runtime overhead of testing a single bit vs. non-zero in the 
> > parameter shouldn't be an issue.
> > 
> 
> The recent introduction of accept4(), signalfd4(), eventfd2(),
> epoll_create1(), dup3(), pipe2(), and inotify_init1() suggest that this
> is the kind of thing you want to plan for, because you're probably going
> to end up doing it anyway.

Yes, that's a very convincing argument. OK, I'll cook something for v6.

Thanks a lot,

Mathieu

> 
> -- 
> Nicholas Miell <nmiell@...cast.net>
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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