linux-kernel - Re: [PATCH] driver core: Fix PM-runtime for links added during consumer probe

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Date:   Tue, 19 Feb 2019 18:07:04 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Jon Hunter <jonathanh@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Lukas Wunner <lukas@...ner.de>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Lucas Stach <l.stach@...gutronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] driver core: Fix PM-runtime for links added during
 consumer probe

On Tue, Feb 19, 2019 at 05:53:26PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit 4c06c4e6cf63 ("driver core: Fix possible supplier PM-usage
> counter imbalance") introduced a regression that causes suppliers
> to be suspended prematurely for device links added during consumer
> driver probe if the initial PM-runtime status of the consumer is
> "suspended" and the consumer is resumed after adding the link and
> before pm_runtime_put_suppliers() is called.  In that case,
> pm_runtime_put_suppliers() will drop the rpm_active refcount for
> the link by one and (since rpm_active is equal to two after the
> preceding consumer resume) the supplier's PM-runtime usage counter
> will be decremented, which may cause the supplier to suspend even
> though the consumer's PM-runtime status is "active".
> 
> For this reason, partially revert commit 4c06c4e6cf63 as the problem
> it tried to fix needs to be addressed somewhat differently, and
> change pm_runtime_get_suppliers() and pm_runtime_put_suppliers() so
> that the latter only drops rpm_active references acquired by the
> former.  [This requires adding a new field to struct device_link,
> but I coulnd't find a cleaner way to address the issue that would
> work in all cases.]
> 
> This causes pm_runtime_put_suppliers() to effectively ignore device
> links added during consumer probe, so device_link_add() doesn't need
> to worry about ensuring that suppliers will remain active after
> pm_runtime_put_suppliers() for links created with DL_FLAG_RPM_ACTIVE
> set and it only needs to bump up rpm_active by one for those links,
> so pm_runtime_active_link() is not necessary any more.
> 
> Fixes: 4c06c4e6cf63 ("driver core: Fix possible supplier PM-usage counter imbalance")
> Reported-by: Jon Hunter <jonathanh@...dia.com>
> Tested-by: Jon Hunter <jonathanh@...dia.com>
> Tested-by: Ulf Hansson <ulf.hansson@...aro.org>
> Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> Changes from the RFC version:
>  - T-by and R-by tags from Jon and Ulf.
>  - More details in the changelog.
> 
> ---
>  drivers/base/core.c          |    4 ++--
>  drivers/base/power/runtime.c |   29 ++++++-----------------------
>  include/linux/device.h       |    1 +
>  include/linux/pm_runtime.h   |    4 ----
>  4 files changed, 9 insertions(+), 29 deletions(-)

I'm sure Jon and Ulf have done much more extensive testing, but I've had
this applied for most of today and haven't run into any issues, so:

Tested-by: Thierry Reding <treding@...dia.com>

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