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Date:   Thu, 16 Sep 2021 10:36:10 +0200
From:   Petr Mladek <pmladek@suse.com>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     linux-kernel@vger.kernel.org, Sumit Garg <sumit.garg@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Julien Thierry <jthierry@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wang Qing <wangqing@vivo.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Santosh Sivaraj <santosh@fossix.org>
Subject: Re: [PATCH 3/5] kernel/watchdog: adapt the watchdog_hld interface
 for async model
Message-ID: <YUMB+ghuYiuhNm2D@alley>
References: <20210915035103.15586-1-kernelfans@gmail.com>
 <20210915035103.15586-4-kernelfans@gmail.com>
 <YUMAUE5RFJAtAS/z@alley>
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On Thu 2021-09-16 10:29:05, Petr Mladek wrote:
> On Wed 2021-09-15 11:51:01, Pingfan Liu wrote:
> > When lockup_detector_init()->watchdog_nmi_probe(), PMU may be not ready
> > yet. E.g. on arm64, PMU is not ready until
> > device_initcall(armv8_pmu_driver_init).  And it is deeply integrated
> > with the driver model and cpuhp. Hence it is hard to push this
> > initialization before smp_init().
> > 
> > But it is easy to take an opposite approach by enabling watchdog_hld to
> > get the capability of PMU async.
> 
> This is another cryptic description. I have probably got it after
> looking at the 5th patch (was not Cc :-(
> 
> > The async model is achieved by introducing an extra parameter notifier
> > of watchdog_nmi_probe().
> 
> I would say that the code is horrible and looks too complex.
> 
> What about simply calling watchdog_nmi_probe() and
> lockup_detector_setup() once again when watchdog_nmi_probe()
> failed in lockup_detector_init()?
> 
> Or do not call lockup_detector_init() at all in
> kernel_init_freeable() when PMU is not ready yet.

BTW: It is an overkill to create your own kthread just to run some
code just once. And you implemeted it a wrong way. The kthread
must wait in a loop until someone else stop it and read
the exit code.

The easiest solution is to queue a work into system_wq for this.

I was not Cc for the 5th patch, so I write it here.

Best Regards,
Petr