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Date: Wed, 4 May 2016 15:53:29 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>, Jessica Yu <jeyu@redhat.com>,
 Jiri Kosina <jikos@kernel.org>, Miroslav Benes <mbenes@suse.cz>,
 Ingo Molnar <mingo@redhat.com>, Michael Ellerman <mpe@ellerman.id.au>,
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 x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
 linux-s390@vger.kernel.org, Vojtech Pavlik <vojtech@suse.com>,
 Jiri Slaby <jslaby@suse.cz>, Chris J Arges <chris.j.arges@canonical.com>,
 Andy Lutomirski <luto@kernel.org>
Subject: Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a
 per-task consistency model
Message-ID: <20160504135329.GQ3430@twins.programming.kicks-ass.net>
References: <cover.1461875890.git.jpoimboe@redhat.com>
 <d60b2b9ce6911bb10dbbf244df58df31c7a8af66.1461875890.git.jpoimboe@redhat.com>
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On Wed, May 04, 2016 at 02:39:40PM +0200, Petr Mladek wrote:
> > +	 * This barrier also ensures that if another CPU goes through the
> > +	 * syscall barrier, sees the TIF_PATCH_PENDING writes in
> > +	 * klp_start_transition(), and calls klp_patch_task(), it also sees the
> > +	 * above write to the target state.  Otherwise it can put the task in
> > +	 * the wrong universe.
> > +	 */
> 
> By other words, it makes sure that klp_patch_task() will assign the
> right patch_state. Where klp_patch_task() could not be called
> before we set TIF_PATCH_PENDING in klp_start_transition().
> 
> > +	smp_wmb();
> > +}

So I've not read the patch; but ending a function with an smp_wmb()
feels wrong.

A wmb orders two stores, and I feel both stores should be well visible
in the same function.