linux-kernel - Re: [PATCH v4 1/2] driver core: Introduce device_link_wait

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Date: Wed, 6 Mar 2024 16:24:47 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Rob Herring
 <robh+dt@...nel.org>, Frank Rowand <frowand.list@...il.com>, Saravana
 Kannan <saravanak@...gle.com>, Lizhi Hou <lizhi.hou@....com>, Max Zhen
 <max.zhen@....com>, Sonal Santan <sonal.santan@....com>, Stefano Stabellini
 <stefano.stabellini@...inx.com>, Jonathan Cameron
 <Jonathan.Cameron@...wei.com>, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, Allan Nielsen <allan.nielsen@...rochip.com>,
 Horatiu Vultur <horatiu.vultur@...rochip.com>, Steen Hegelund
 <steen.hegelund@...rochip.com>, Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Nuno Sa <nuno.sa@...log.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, stable@...r.kernel.org
Subject: Re: [PATCH v4 1/2] driver core: Introduce
 device_link_wait_removal()

Hi Rafael,

On Wed, 6 Mar 2024 13:48:37 +0100
"Rafael J. Wysocki" <rafael@...nel.org> wrote:

> On Wed, Mar 6, 2024 at 9:51 AM Herve Codina <herve.codina@...tlin.com> wrote:
> >
> > The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > introduces a workqueue to release the consumer and supplier devices used
> > in the devlink.
> > In the job queued, devices are release and in turn, when all the
> > references to these devices are dropped, the release function of the
> > device itself is called.
> >
> > Nothing is present to provide some synchronisation with this workqueue
> > in order to ensure that all ongoing releasing operations are done and
> > so, some other operations can be started safely.
> >
> > For instance, in the following sequence:
> >   1) of_platform_depopulate()
> >   2) of_overlay_remove()
> >
> > During the step 1, devices are released and related devlinks are removed
> > (jobs pushed in the workqueue).
> > During the step 2, OF nodes are destroyed but, without any
> > synchronisation with devlink removal jobs, of_overlay_remove() can raise
> > warnings related to missing of_node_put():
> >   ERROR: memory leak, expected refcount 1 instead of 2
> >
> > Indeed, the missing of_node_put() call is going to be done, too late,
> > from the workqueue job execution.
> >
> > Introduce device_link_wait_removal() to offer a way to synchronize
> > operations waiting for the end of devlink removals (i.e. end of
> > workqueue jobs).
> > Also, as a flushing operation is done on the workqueue, the workqueue
> > used is moved from a system-wide workqueue to a local one.
> >
> > Fixes: 80dd33cf72d1 ("drivers: base: Fix device link removal")  
> 
> No, it is not fixed by this patch.

Was explicitly asked by Saravana on v1 review:
https://lore.kernel.org/linux-kernel/CAGETcx9uP86EHyKJNifBMd23oCsA+KpMa+e36wJEEnHDve+Avg@mail.gmail.com/

The commit 80dd33cf72d1 introduces the workqueue and so some asynchronous tasks
on removal.
This patch and the next one allows to re-sync execution waiting for jobs in
the workqueue when it is needed.

> 
> In fact, the only possibly observable effect of this patch is the
> failure when the allocation of device_link_wq fails AFAICS.
> 
> > Cc: stable@...r.kernel.org  
> 
> So why?

Cc:stable is needed as this patch is a prerequisite of patch 2 (needed
to fix the asynchronous workqueue task issue).

Best regards,
Hervé

-- 
Hervé Codina, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com