linux-kernel - Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <6F943352-C454-4533-B61C-DF7D828B3B7D@zytor.com>
Date:	Mon, 29 Feb 2016 10:19:17 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ben Maurer <bmaurer@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	linux-api <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dave Watson <davejwatson@...com>,
	rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Chris Lameter <cl@...ux.com>, Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Paul Turner <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Hunter <ahh@...gle.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

On February 29, 2016 4:41:49 AM PST, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>
>Agreed that many architectures issue slower instructions when reading
>from packed structures, which is unwanted.
>

And detrimental to atomicity.

>Could we require that each field be naturally aligned and require that
>they are placed so _no_ padding whatsoever should ever be added by the
>compiler ? If that's possible, then we could remove the packed.

What people have been trying to tell you is that we *must* do this, and no compiler truck like packed will help.


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