Re: [PATCH 4.5 142/238] watchdog: dont run proc_watchdog_update if new value is same as old

[Ben Hutchings]

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

> 4.5-stable review patch.ÂÂIf anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Joshua Hunt <johunt@xxxxxxxxxx>
> 
> 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().
> 
> Â NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> Â NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> Â NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> Â 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@xxxxxxxxxx>
> Acked-by: Don Zickus <dzickus@xxxxxxxxxx>
> Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
> Cc: Ulrich Obergfell <uobergfe@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> ---
> Âkernel/watchdog.c |ÂÂÂÂ9 ++++++++-
> Â1 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
> Âint proc_watchdog_thresh(struct ctl_table *table, int write,
> Â			Âvoid __user *buffer, size_t *lenp, loff_t *ppos)
> Â{
> -	int err, old;
> +	int err, old, new;
> Â
> Â	get_online_cpus();
> Â	mutex_lock(&watchdog_proc_mutex);
> @@ -987,6 +990,10 @@ int proc_watchdog_thresh(struct ctl_tabl
> Â	/*
> Â	Â* Update the sample period. Restore on failure.
> Â	Â*/
> +	new = ACCESS_ONCE(watchdog_thresh);

This ACCESS_ONCE() doesn't make any sense to me. ÂIsn't watchdog_thresh
protected by watchdog_proc_mutex? ÂIf 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;
> +
> Â	set_sample_period();
> Â	err = proc_watchdog_update();
> Â	if (err) {

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

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