linux-kernel - Re: [RFC PATCH] printk: Use ACCESS_ONCE() instead of a volatile type

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Date:	Thu, 13 Nov 2014 23:57:22 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alex Elder <elder@...aro.org>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>, Jan Kara <jack@...e.cz>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Joe Perches <joe@...ches.com>,
	open list <linux-kernel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [RFC PATCH] printk: Use ACCESS_ONCE() instead of a volatile
 type

On Thu, 13 Nov 2014 22:48:33 -0600
Alex Elder <elder@...aro.org> wrote:

> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index e748971..4790191 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1624,7 +1624,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >  	int printed_len = 0;
> >  	bool in_sched = false;
> >  	/* cpu currently holding logbuf_lock in this function */
> > -	static volatile unsigned int logbuf_cpu = UINT_MAX;
> > +	static unsigned int logbuf_cpu = UINT_MAX;
> 
> If this is not volatile, can the compiler assume that it
> can't change before the first access?  Put another way,
> does this assignment need to be done more like this?
> 
> 	static unsigned int ACCESS_ONCE(logbuf_cpu) = UINT_MAX;
> 
> (I haven't checked, but I don't believe that expands to valid code.)
> 

I can bet you that it doesn't compile.

That assignment is what it is initialized to at boot up. I can't see
any optimization that would cause gcc to modify that. Especially since
we are hiding its accesses within the ACCESS_ONCE(). That alone should
confuse gcc enough to leave it a hell alone J.

-- Steve
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