linux-kernel - Re: [PATCH 4.5 142/238] watchdog: dont run proc_watchdog

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Date:	Tue, 12 Apr 2016 23:41:43 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Cc:	stable@...r.kernel.org, Josh Hunt <johunt@...mai.com>,
	Don Zickus <dzickus@...hat.com>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4.5 142/238] watchdog: dont run proc_watchdog_update if
 new value is same as old

On Sun, 2016-04-10 at 11:35 -0700, Greg Kroah-Hartman wrote:

> 4.5-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Joshua Hunt <johunt@...mai.com>
> 
> commit a1ee1932aa6bea0bb074f5e3ced112664e4637ed upstream.
> 
> While working on a script to restore all sysctl params before a series of
> tests I found that writing any value into the
> /proc/sys/kernel/{nmi_watchdog,soft_watchdog,watchdog,watchdog_thresh}
> causes them to call proc_watchdog_update().
> 
>   NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
>   NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
>   NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
>   NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> 
> There doesn't appear to be a reason for doing this work every time a write
> occurs, so only do it when the values change.
> 
> Signed-off-by: Josh Hunt <johunt@...mai.com>
> Acked-by: Don Zickus <dzickus@...hat.com>
> Reviewed-by: Aaron Tomlin <atomlin@...hat.com>
> Cc: Ulrich Obergfell <uobergfe@...hat.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> ---
>  kernel/watchdog.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
[...]
> @@ -967,7 +970,7 @@ int proc_soft_watchdog(struct ctl_table
>  int proc_watchdog_thresh(struct ctl_table *table, int write,
>  			 void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
> -	int err, old;
> +	int err, old, new;
>  
>  	get_online_cpus();
>  	mutex_lock(&watchdog_proc_mutex);
> @@ -987,6 +990,10 @@ int proc_watchdog_thresh(struct ctl_tabl
>  	/*
>  	 * Update the sample period. Restore on failure.
>  	 */
> +	new = ACCESS_ONCE(watchdog_thresh);

This ACCESS_ONCE() doesn't make any sense to me.  Isn't watchdog_thresh
protected by watchdog_proc_mutex?  If a race on watchdog_thresh is
still possible then the check for old == new isn't a valid
optimisation, and if it isn't possible then ACCESS_ONCE() shouldn't be
used here.

Ben.

> +	if (old == new)
> +		goto out;
> +
>  	set_sample_period();
>  	err = proc_watchdog_update();
>  	if (err) {

-- 
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.
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