linux-kernel - Re: [PATCH] printk: drop logbuf

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Message-ID: <20141118091022.GA19318@quack.suse.cz>
Date:	Tue, 18 Nov 2014 10:10:22 +0100
From:	Jan Kara <jack@...e.cz>
To:	Alex Elder <elder@...aro.org>
Cc:	akpm@...ux-foundation.org, bobby.prani@...il.com, pmladek@...e.cz,
	jack@...e.cz, mcgrof@...e.com, rostedt@...dmis.org,
	joe@...ches.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: drop logbuf_cpu volatile qualifier

On Fri 14-11-14 13:19:31, Alex Elder wrote:
> Pranith Kumar posted a patch in which removed the "volatile"
> qualifier for the "logbuf_cpu" variable in vprintk_emit().
>     https://lkml.org/lkml/2014/11/13/894
> In his patch, he used ACCESS_ONCE() for all references to
> that symbol to provide whatever protection was intended.
> 
> There was some discussion that followed, and in the end
> Stephen Rostedt concluded that not only was "volatile" not
> needed, neither was it required to use ACCESS_ONCE().  I
> offered an elaborate description that concluded Stephen
> was right, and Pranith asked me to submit an alternative
> patch.  And this is it.
> 
> The basic reason "volatile" is not needed is that "logbuf_cpu" has
> static storage duration, and vprintk_emit() is an exported
> interface.  This means that the value of logbuf_cpu must be read
> from memory the first time it is used in a particular call of
> vprintk_emit().  The variable's value is read only once in that
> function, when it's read it'll be the copy from memory (or cache).
> 
> In addition, the value of "logbuf_cpu" is only ever written under
> protection of a spinlock.  So the value that is read is the "real"
> value (and not an out-of-date cached one).  If its value is not
> UINT_MAX, it is the current CPU's processor id, and it will have
> been last written by the running CPU.
> 
> Reported-by: Pranith Kumar <bobby.prani@...il.com>
> Fix-suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Alex Elder <elder@...aro.org>
> ---
> 	Note!!!  I am not able to stress test this patch.
  The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>

  BTW, stress-testing this is tough. I never saw printk recursion happening
in practice - only when I screwed up something in printk code when
experimenting.

								Honza

>  kernel/printk/printk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ced2b84..fefc8d0 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1627,7 +1627,7 @@ 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 (level == SCHED_MESSAGE_LOGLEVEL) {
>  		level = -1;
> -- 
> 1.9.1
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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