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

[Rafael J. Wysocki]

On Thu, Feb 29, 2024 at 2:03 PM Nuno Sá <noname.nuno@xxxxxxxxx> wrote:
>
> On Thu, 2024-02-29 at 14:01 +0100, Rafael J. Wysocki wrote:
> > On Thu, Feb 29, 2024 at 12:13 PM Nuno Sá <noname.nuno@xxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> > > > ---
> > > >  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.
>
>
> Yeah, that's also my understanding...

Moreover, IIUC this is called after dropping the last reference to the
device link in question and so after queuing up the link removal work.
Because that work does not requeue itself, flush_workqueue() is
sufficient to ensure that the removal work has been completed.

If anyone thinks that it may not be sufficient, please explain to me
why you think so.  Otherwise, don't do stuff to prevent things you
cannot explain.