linux-kernel - Re: [PATCH v14 for 4.1] sys_membarrier(): system-wide memory barrier (x86)

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Date:	Tue, 14 Apr 2015 21:35:57 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v14 for 4.1] sys_membarrier(): system-wide memory barrier
 (x86)

On Tue, 14 Apr 2015, Mathieu Desnoyers wrote:
> Thinking about it a bit more, one reason for doing the QUERY along
> with the exact set of flags queried allow us to do more than just
> returning which flags are supported: it allows us to tell userspace
> whether the combination of flags used is valid or not.
> 
> For instance, if we add a MEMBARRIER_PRIVATE flag in a future release
> to issue memory barriers only to other threads from the same process,
> and we add a MEMBARRIER_EXPEDITED which uses IPIs to issue those
> barriers, we could very well have a situation where using
> 
>   EXPEDITED | PRIVATE  would be valid (only sending IPIs to CPUs
>   running threads from the same process)
> 
> but
> 
>   EXPEDITED alone would be invalid (-EINVAL), until we figure out
>   how to expedite memory barriers to all processors without impacting
>   other processes, if at all possible.
> 
> Using QUERY with an empty set of flags could however return the set of
> flags supported, which could be a nice feature. Anyway, I think
> the "0" flag should be the basic always correct configuration that
> is always supported, otherwise we'd have -ENOSYS. Therefore, querying
> whether the empty set of flags is supported has little value, other
> than checking for -ENOSYS.
> 
> So considering the above, the typical use of this query method from
> library initialization would be:
> 
> int supported_flags = sys_membarrier(MEMBARRIER_QUERY);
> 
> ... check for -ENOSYS ....
> ... check whether the flags we need are supported ...
> 
> if (sys_membarrier(MEMBARRIER_QUERY | flag1 | flag2))
>   goto error;
> 
> then we are guaranteed that using sys_membarrier(flag1 | flag2)
> will always succeed within the application, without needing to
> handle errors every time it is used. This property is useful
> to implement a synchronize_rcu() that returns "void" and simplify
> error handling within the application.

So how many of these "flags" are you planning to implement and how
many valid combinations are going to exist?

I doubt it's more than a dozen. So I prefer explicit operation modes
for the valid ones rather than having a random pile of "flags".

Thanks,

	tglx


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