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Subject: Re: Jump Label initialization
From: Jan Glauber <jang@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Jason Baron <jbaron@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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Date: Thu, 29 Sep 2011 15:10:26 +0200
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On Thu, 2011-09-29 at 08:40 -0400, Steven Rostedt wrote:
> On Thu, 2011-09-29 at 14:04 +0200, Jan Glauber wrote:
> > On Wed, 2011-09-28 at 19:14 -0700, Jeremy Fitzhardinge wrote:
> > > Hi all,
> > > 
> > > I'm trying to use the jump label machinery as part of the pv ticketlock
> > > work I'm doing on x86.
> > > 
> > > The problem I'm having at the moment is that I do my spinlock setup in
> > > smp_prepare_boot_cpu(), which happens before jump_label_init() gets
> > > called, and so the latter goes and nops out all my enabled jump label key.
> > > 
> > > I'm experimenting at the moment with a patch to allow
> > > jump_label_enable() to be called fairly early, and have that be
> > > respected by jump_label_init().  I'm doing this by replacing
> > > arch_jump_label_poke_text_early() with
> > > arch_jump_label_transform_early(), which shares most of its code with
> > > its non-early variant, except that it expects to run in a pre-SMP
> > > environment.
> > > 
> > > Does this seem plausible? (I haven't tested it yet.)
> > > 
> > > The x86, mips and sparc patches are fairly simple; I forgot to look at
> > > powerpc, and I didn't fully investigate s390.
> > 
> > s390 does not have the early() variant since it didn't need it. On
> > pre-SMP we probably don't need stop_machine() so creating
> > arch_jump_label_transform_early() by leaving out stop_machine()
> > and patching the code directly should be fine.
> 
> Also note that stop_machine() may only be needed by a few archs (maybe
> only x86).

s390 needs stop_machine() too. Instructions may be fetched multiple
times for a single execution and may also be fetched piecemeal.

--Jan