From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread
Date: Tue, 1 Mar 2016 22:36:11 +0100
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To: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, 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>
List-Id: linux-api@vger.kernel.org

On Tue, Mar 01, 2016 at 10:32:02PM +0100, Peter Zijlstra wrote:

> >         /*
> >          * Thread-local ABI rseq_seqnum field.
> >          * Updated by the kernel, and read by user-space with
> >          * single-copy atomicity semantics. Aligned on 32-bit.
> >          * Values:
> >          * >= 0: current seqnum for this thread (feature is active).
> >          * -1 (initial value): means the rseq feature is inactive.
> >          * -2: rseq feature is not available.
> >          */
> >         int32_t rseq_seqnum;
> 
> So I really hate that, that makes we have to check for these special
> values whenever we increment the seq count and cannot have it wrap
> naturally.

Also, since it will wrap, uint32_t is more natural, since the whole
signed overflow thing is somewhat undefined in C.