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

[Thierry Reding]

On Tue, Feb 19, 2019 at 05:53:26PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> Tested-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 
> 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@xxxxxxxxxx>

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