linux-kernel - Re: [PATCH v3] PM-runtime: fix deadlock with ktime

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Date:   Wed, 30 Jan 2019 22:53:00 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Biju Das <biju.das@...renesas.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v3] PM-runtime: fix deadlock with ktime

On Wed, Jan 30, 2019 at 6:26 PM Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> A deadlock has been seen when swicthing clocksources which use PM runtime.
> The call path is:
> change_clocksource
>     ...
>     write_seqcount_begin
>     ...
>     timekeeping_update
>         ...
>         sh_cmt_clocksource_enable
>             ...
>             rpm_resume
>                 pm_runtime_mark_last_busy
>                     ktime_get
>                         do
>                             read_seqcount_begin
>                         while read_seqcount_retry
>     ....
>     write_seqcount_end
>
> Although we should be safe because we haven't yet changed the clocksource
> at that time, we can't because of seqcount protection.
>
> Use ktime_get_mono_fast_ns() instead which is lock safe for such case
>
> With ktime_get_mono_fast_ns, the timestamp is not guaranteed to be
> monotonic across an update and as a result can goes backward. According to
> update_fast_timekeeper() description: "In the worst case, this can
> result is a slightly wrong timestamp (a few nanoseconds)". For
> PM runtime autosuspend, this means only that the suspend decision can
> be slightly sub optimal.
>
> Fixes: 8234f6734c5d ("PM-runtime: Switch autosuspend over to using hrtimers")
> Reported-by: Biju Das <biju.das@...renesas.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>
> Hi Rafael,
>
> Sorry, I sent the version with the typo mistake that generated the compilation error
> reported by kbuild-test-robot
>
> This version doesn't have the typo.

OK, I've applied this one, thanks!