Re: [PATCH 0/4] watchdog: avoid races in watchdog_nmi_{en|disable} functions

[Aaron Tomlin <atomlin@redhat.com>]

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On Sat 2015-08-01 14:49 +0200, Ulrich Obergfell wrote:
> Originally watchdog_nmi_enable(cpu) and watchdog_nmi_disable(cpu) were
> only called in watchdog thread context. However, the following commits
> utilize these functions outside of watchdog thread context too.
> 
>   commit 9809b18fcf6b8d8ec4d3643677345907e6b50eca
>   Author: Michal Hocko <mhocko@suse.cz>
>   Date:   Tue Sep 24 15:27:30 2013 -0700
> 
>       watchdog: update watchdog_thresh properly
> 
>   commit b3738d29323344da3017a91010530cf3a58590fc
>   Author: Stephane Eranian <eranian@google.com>
>   Date:   Mon Nov 17 20:07:03 2014 +0100
> 
>       watchdog: Add watchdog enable/disable all functions
> 
> Hence, it is now possible that these functions execute concurrently with
> the same 'cpu' argument. This concurrency is problematic because per-cpu
> 'watchdog_ev' can be accessed/modified without adequate synchronization.
> 
> The patch series aims to address the above problem. However, instead of
> introducing locks to protect per-cpu 'watchdog_ev' a different approach
> is taken: Invoke these functions by parking and unparking the watchdog
> threads (to ensure they are always called in watchdog thread context).
> 
>   static struct smp_hotplug_thread watchdog_threads = {
>            ...
>           .park   = watchdog_disable, // calls watchdog_nmi_disable()
>           .unpark = watchdog_enable,  // calls watchdog_nmi_enable()
>   };
> 
> Both previously mentioned commits call these functions in a similar way
> and thus in principle contain some duplicate code. The patch series also
> avoids this duplication by providing a commonly usable mechanism.
> 
> - Patch 1/4 introduces the watchdog_{park|unpark}_threads functions that
>   park/unpark all watchdog threads specified in 'watchdog_cpumask'. They
>   are intended to be called inside of kernel/watchdog.c only.
> 
> - Patch 2/4 introduces the watchdog_{suspend|resume} functions which can
>   be utilized by external callers to deactivate the hard and soft lockup
>   detector temporarily.
> 
> - Patch 3/4 utilizes watchdog_{park|unpark}_threads to replace some code
>   that was introduced by commit 9809b18fcf6b8d8ec4d3643677345907e6b50eca.
> 
> - Patch 4/4 utilizes watchdog_{suspend|resume} to replace some code that
>   was introduced by commit b3738d29323344da3017a91010530cf3a58590fc.
> 
> A few corner cases should be mentioned here for completeness.
> 
> - kthread_park() of watchdog/N could hang if cpu N is already locked up.
>   However, if watchdog is enabled the lockup will be detected anyway.
> 
> - kthread_unpark() of watchdog/N could hang if cpu N got locked up after
>   kthread_park(). The occurrence of this scenario should be _very_ rare
>   in practice, in particular because it is not expected that temporary
>   deactivation will happen frequently, and if it happens at all it is
>   expected that the duration of deactivation will be short.
> 
> Ulrich Obergfell (4):
>   watchdog: introduce watchdog_park_threads() and
>     watchdog_unpark_threads()
>   watchdog: introduce watchdog_suspend() and watchdog_resume()
>   watchdog: use park/unpark functions in update_watchdog_all_cpus()
>   watchdog: use suspend/resume interface in fixup_ht_bug()
> 
>  arch/x86/kernel/cpu/perf_event_intel.c |   9 +-
>  include/linux/nmi.h                    |   2 +
>  include/linux/watchdog.h               |   8 --
>  kernel/watchdog.c                      | 157 +++++++++++++++++++--------------
>  4 files changed, 100 insertions(+), 76 deletions(-)
> 

The whole patch series looks good to me.

Reviewed-by: Aaron Tomlin <atomlin@redhat.com>

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