Re: [PATCH 1/2] driver: core: add dedicated workqueue for devlink removal

[Rafael J. Wysocki]

On Mon, Feb 5, 2024 at 9:29 AM Nuno Sá <noname.nuno@xxxxxxxxx> wrote:
>
> On Fri, 2024-02-02 at 16:59 +0100, Rafael J. Wysocki wrote:
> > On Fri, Feb 2, 2024 at 1:18 PM Nuno Sa via B4 Relay
> > <devnull+nuno.sa.analog.com@xxxxxxxxxx> wrote:
> > >
> > > From: Nuno Sa <nuno.sa@xxxxxxxxxx>
> > >
> > > Let's use a dedicated queue for devlinks since releasing a link happens
> > > asynchronously but some code paths, like DT overlays, have some
> > > expectations regarding the of_node when being removed (the refcount must
> > > be 1). Given how devlinks are released that cannot be assured. Hence, add a
> > > dedicated queue so that it's easy to sync against devlinks removal.
> >
> > Thanks for following my suggestion!
> >
> > > While at it, make sure to explicitly include <linux/workqueue.h>.
> > >
> > > Signed-off-by: Nuno Sa <nuno.sa@xxxxxxxxxx>
> > > ---
> > >  drivers/base/core.c    | 33 +++++++++++++++++++++++++++++----
> > >  include/linux/fwnode.h |  1 +
> > >  2 files changed, 30 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 14d46af40f9a..06e7766b5227 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -31,6 +31,7 @@
> > >  #include <linux/swiotlb.h>
> > >  #include <linux/sysfs.h>
> > >  #include <linux/dma-map-ops.h> /* for dma_default_coherent */
> > > +#include <linux/workqueue.h>
> > >
> > >  #include "base.h"
> > >  #include "physical_location.h"
> > > @@ -44,6 +45,7 @@ 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 *devlink_release_queue __ro_after_init;
> > >
> > >  /**
> > >   * __fwnode_link_add - Create a link between two fwnode_handles.
> > > @@ -235,6 +237,11 @@ static void __fw_devlink_pickup_dangling_consumers(struct
> > > fwnode_handle *fwnode,
> > >                 __fw_devlink_pickup_dangling_consumers(child, new_sup);
> > >  }
> > >
> > > +void fwnode_links_flush_queue(void)
> > > +{
> > > +       flush_workqueue(devlink_release_queue);
> > > +}
> > > +
> > >  static DEFINE_MUTEX(device_links_lock);
> > >  DEFINE_STATIC_SRCU(device_links_srcu);
> > >
> > > @@ -531,9 +538,10 @@ 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.
> > > +        * devlink workqueue.
> > > +        *
> > >          */
> > > -       queue_work(system_long_wq, &link->rm_work);
> > > +       queue_work(devlink_release_queue, &link->rm_work);
> > >  }
> > >
> > >  static struct class devlink_class = {
> > > @@ -636,10 +644,27 @@ static int __init devlink_class_init(void)
> > >                 return ret;
> > >
> > >         ret = class_interface_register(&devlink_class_intf);
> > > -       if (ret)
> > > +       if (ret) {
> > > +               class_unregister(&devlink_class);
> > > +               return ret;
> > > +       }
> > > +
> > > +       /*
> > > +        * Using a dedicated queue for devlinks since releasing a link happens
> > > +        * asynchronously but some code paths, like DT overlays, have some
> > > +        * expectations regarding the of_node when being removed (the refcount
> > > +        * must be 1). Given how devlinks are released that cannot be assured.
> > > +        * Hence, add a dedicated queue so that it's easy to sync against
> > > +        * devlinks removal.
> > > +        */
> > > +       devlink_release_queue = alloc_workqueue("devlink_release", 0, 0);
> > > +       if (!devlink_release_queue) {
> > > +               class_interface_unregister(&devlink_class_intf);
> > >                 class_unregister(&devlink_class);
> >
> > This is a bit drastic.
> >
> > I think that device links can still work if devlink_release_queue is
> > NULL, just devlink_dev_release() needs to check it and release
> > synchronously if it is NULL.
> >
>
> Agreed, I'll do that way. It will always synchronously remove the links (which is
> different than before) but I guess that failing in allocating the queue is rather
> unlikely.

Right.