From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread
Date: Mon, 29 Feb 2016 11:39:34 +0100
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To: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Cc: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Linux Kernel Mailing List <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
List-Id: linux-api@vger.kernel.org

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