Re: suspend regression in 4.1-rc1

[Ulrich Obergfell <uobergfe@redhat.com>]

Peter,

please see my comments in-line.

Regards,

Uli

----- Original Message -----
From: "Peter Zijlstra" <peterz@infradead.org>
To: "Michal Hocko" <mhocko@suse.cz>
[...]
> On Mon, May 18, 2015 at 11:03:37AM +0200, Michal Hocko wrote:
>> This doesn't hang anymore. I've just had to move the mutex definition
>> up to make it compile. So feel free to add my
>
> I've also fixed a lock leak, see goto unlock :-)
>
>> Reported-and-tested-by: Michal Hocko <mhocko@suse.cz>
>
> *blink* that actually fixed it..
>
> That somewhat leaves me at a loss explaining how s2r was failing.
>
> ---
> Subject: watchdog: Fix merge 'conflict'
>
> Two watchdog changes that came through different trees had a non
> conflicting conflict, that is, one changed the semantics of a variable
> but no actual code conflict happened. So the merge appeared fine, but
> the resulting code did not behave as expected.
>
> Commit 195daf665a62 ("watchdog: enable the new user interface of the
> watchdog mechanism") changes the semantics of watchdog_user_enabled,
> which thereafter is only used by the functions introduced by
> b3738d293233 ("watchdog: Add watchdog enable/disable all functions").

Don and I already posted a patch in April to address this:

https://lkml.org/lkml/2015/4/22/306
http://ozlabs.org/~akpm/mmots/broken-out/watchdog-fix-watchdog_nmi_enable_all.patch

> There further appears to be a distinct lack of serialization between
> setting and using watchdog_enabled, so perhaps we should wrap the
> {en,dis}able_all() things in watchdog_proc_mutex.

As I understand it, the {en,dis}able_all() functions are only called early
at kernel startup, so I do not see how they could be racing with watchdog
code that is executed in the context of write() system calls to parameters
in /proc/sys/kernel. Please see also my earlier reply to Michal for further
details: http://marc.info/?l=linux-pm&m=143194387208250&w=2

Do we really need synchronization here?

> This patch fixes a s2r failure reported by Michal; which I cannot
> readily explain. But this does make the code internally consistent
> again.
>
> Reported-and-tested-by: Michal Hocko <mhocko@suse.cz>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/watchdog.c |   20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 2316f50..506edcc5 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -41,6 +41,8 @@
>  #define NMI_WATCHDOG_ENABLED      (1 << NMI_WATCHDOG_ENABLED_BIT)
>  #define SOFT_WATCHDOG_ENABLED     (1 << SOFT_WATCHDOG_ENABLED_BIT)
> 
> +static DEFINE_MUTEX(watchdog_proc_mutex);
> +
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  static unsigned long __read_mostly watchdog_enabled = SOFT_WATCHDOG_ENABLED|NMI_WATCHDOG_ENABLED;
>  #else
> @@ -608,26 +610,36 @@ void watchdog_nmi_enable_all(void)
>  {
>          int cpu;
> 
> -        if (!watchdog_user_enabled)
> -                return;
> +        mutex_lock(&watchdog_proc_mutex);
> +
> +        if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
> +                goto unlock;
> 
>          get_online_cpus();
>          for_each_online_cpu(cpu)
>                  watchdog_nmi_enable(cpu);
>          put_online_cpus();
> +
> +unlock:
> +        mutex_lock(&watchdog_proc_mutex);
>  }
> 
>  void watchdog_nmi_disable_all(void)
>  {
>          int cpu;
> 
> +        mutex_lock(&watchdog_proc_mutex);
> +
>          if (!watchdog_running)
> -                return;
> +                goto unlock;
> 
>          get_online_cpus();
>          for_each_online_cpu(cpu)
>                  watchdog_nmi_disable(cpu);
>          put_online_cpus();
> +
> +unlock:
> +        mutex_unlock(&watchdog_proc_mutex);
>  }
>  #else
>  static int watchdog_nmi_enable(unsigned int cpu) { return 0; }
> @@ -744,8 +756,6 @@ static int proc_watchdog_update(void)
> 
>  }
> 
> -static DEFINE_MUTEX(watchdog_proc_mutex);
> -
>  /*
>   * common function for watchdog, nmi_watchdog and soft_watchdog parameter
>   *