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

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Date:	Wed, 13 Jan 2010 11:53:27 -0800
From:	Nicholas Miell <nmiell@...cast.net>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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)

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. :-)
> 
> 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.

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

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