From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Linux PM list <linux-pm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
"open list:TI ETHERNET SWITCH DRIVER (CPSW)"
<linux-omap@vger.kernel.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Ulf Hansson <ulf.hansson@linaro.org>,
Biju Das <biju.das@bp.renesas.com>,
Linux-Renesas <linux-renesas-soc@vger.kernel.org>
Subject: Re: [PATCH] PM-runtime: fix deadlock with ktime
Date: Wed, 30 Jan 2019 09:21:41 +0100 [thread overview]
Message-ID: <CAMuHMdWYV-tDD2osdvs4yhD-mu2n8-JSC7rV8Zt5vW4kr8hdWQ@mail.gmail.com> (raw)
In-Reply-To: <1548836194-15264-1-git-send-email-vincent.guittot@linaro.org>
Hi Vincent,
On Wed, Jan 30, 2019 at 9:16 AM Vincent Guittot
<vincent.guittot@linaro.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
>
> Fixes: 8234f6734c5d ("PM-runtime: Switch autosuspend over to using hrtimers")
> Reported-by: Biju Das <biju.das@bp.renesas.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Thanks for your patch!
/**
* ktime_get_mono_fast_ns - Fast NMI safe access to clock monotonic
*
* This timestamp is not guaranteed to be monotonic across an update.
* The timestamp is calculated by:
*
* now = base_mono + clock_delta * slope
*
* So if the update lowers the slope, readers who are forced to the
* not yet updated second array are still using the old steeper slope.
*
* tmono
* ^
* | o n
* | o n
* | u
* | o
* |o
* |12345678---> reader order
*
* o = old slope
* u = update
* n = new slope
*
* So reader 6 will observe time going backwards versus reader 5.
*
* While other CPUs are likely to be able observe that, the only way
* for a CPU local observation is when an NMI hits in the middle of
* the update. Timestamps taken from that NMI context might be ahead
* of the following timestamps. Callers need to be aware of that and
* deal with it.
*/
As this function is not guaranteed to be monotonic, have you checked how
the Runtime PM code behaves if time goes backwards? Does it just make
a suboptimal decision or does it crash?
Thanks!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
next prev parent reply other threads:[~2019-01-30 8:21 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-30 8:16 [PATCH] PM-runtime: fix deadlock with ktime Vincent Guittot
2019-01-30 8:21 ` Geert Uytterhoeven [this message]
2019-01-30 9:14 ` Vincent Guittot
2019-01-30 9:39 ` Rafael J. Wysocki
2019-01-30 9:41 ` Vincent Guittot
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