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

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Message-ID: <3364335.Bqf8sAzlTS@wuerfel>
Date:	Mon, 29 Feb 2016 11:39:34 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

On Monday 29 February 2016 11:32:21 Peter Zijlstra wrote:
> On Sun, Feb 28, 2016 at 12:39:54AM +0000, Mathieu Desnoyers wrote:
> 
> > /* This structure needs to be aligned cache line size. */
> > struct thread_local_abi {
> >   int32_t  cpu_id;
> >   uint32_t rseq_seqnum;
> >   uint64_t rseq_post_commit_ip;
> >   /* Add new fields at the end. */ 
> > } __attribute__((packed));
> 
> I would really not use packed; that can lead to horrible layout.
> 
> Suppose someone would add:
> 
> 	uint32_t foo;
> 	uint64_t bar;
> 
> With packed, you get an unaligned uint64_t in there, which is horrible.
> Without packed, you get a hole, which you can later fill.

What's making things worse is that on some architectures, adding
__packed will force access by bytes rather than just reading
a 32-bit or 64-bit numbers directly, so it's slow and non-atomic.

	Arnd