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

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Date:	Wed, 13 Jan 2010 11:42:00 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Nicholas Miell <nmiell@...cast.net>, 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)

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.

David Daney
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