Re: [PATCH v3 1/2] driver core: Introduce device_link_wait_removal()

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Feb 29, 2024 at 12:13 PM Nuno Sá <noname.nuno@gmail.com> wrote:
>
> Hi,
>
> Just copy pasting my previous comments :)
>
> On Thu, 2024-02-29 at 11:52 +0100, Herve Codina 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")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> > ---
> >  drivers/base/core.c    | 26 +++++++++++++++++++++++---
> >  include/linux/device.h |  1 +
> >  2 files changed, 24 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index d5f4e4aac09b..80d9430856a8 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -44,6 +44,7 @@ static bool fw_devlink_is_permissive(void);
> >  static void __fw_devlink_link_to_consumers(struct device *dev);
> >  static bool fw_devlink_drv_reg_done;
> >  static bool fw_devlink_best_effort;
> > +static struct workqueue_struct *device_link_wq;
> >
> >  /**
> >   * __fwnode_link_add - Create a link between two fwnode_handles.
> > @@ -532,12 +533,26 @@ static void devlink_dev_release(struct device *dev)
> >       /*
> >        * It may take a while to complete this work because of the SRCU
> >        * synchronization in device_link_release_fn() and if the consumer or
> > -      * supplier devices get deleted when it runs, so put it into the
> > "long"
> > -      * workqueue.
> > +      * supplier devices get deleted when it runs, so put it into the
> > +      * dedicated workqueue.
> >        */
> > -     queue_work(system_long_wq, &link->rm_work);
> > +     queue_work(device_link_wq, &link->rm_work);
> >  }
> >
> > +/**
> > + * device_link_wait_removal - Wait for ongoing devlink removal jobs to
> > terminate
> > + */
> > +void device_link_wait_removal(void)
> > +{
> > +     /*
> > +      * devlink removal jobs are queued in the dedicated work queue.
> > +      * To be sure that all removal jobs are terminated, ensure that any
> > +      * scheduled work has run to completion.
> > +      */
> > +     drain_workqueue(device_link_wq);
> > +}
>
> I'm still not convinced we can have a recursive call into devlinks removal so I
> do think flush_workqueue() is enough. I will defer to Saravana though...

AFAICS, the difference betwee flush_workqueue() and drain_workqueue()
is the handling of the case when a given work item can queue up itself
again.  This does not happen here.