Re: [PATCH v2] driver core: Allow device link operations inside sync_state()

[Rafael J. Wysocki]

On Thu, Nov 14, 2019 at 10:59 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> On Thu, Nov 14, 2019 at 1:56 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 14, 2019 at 8:42 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
> > >
> > > Some sync_state() implementations might need to call APIs that in turn
> > > make calls to device link APIs. So, do the sync_state() callbacks
> > > without holding the device link lock.
> > >
> > > Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> >
> > It would have been kind of nice to let me respond to your last reply
> > before sending this.  Oh well.
> >
> > > ---
> > >  drivers/base/core.c | 76 ++++++++++++++++++++++++++++++++++++++++-----
> > >  1 file changed, 68 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index e6d3e6d485da..2f14d4bf1472 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -48,6 +48,8 @@ early_param("sysfs.deprecated", sysfs_deprecated_setup);
> > >  static LIST_HEAD(wait_for_suppliers);
> > >  static DEFINE_MUTEX(wfs_lock);
> > >  static LIST_HEAD(deferred_sync);
> > > +static LIST_HEAD(sync_list);
> > > +static DEFINE_MUTEX(sync_lock);
> >
> > The two items above are not needed AFAICS.
> >
> > >  static unsigned int defer_sync_state_count = 1;
> > >
> > >  #ifdef CONFIG_SRCU
> > > @@ -695,7 +697,23 @@ int device_links_check_suppliers(struct device *dev)
> > >         return ret;
> > >  }
> > >
> > > -static void __device_links_supplier_sync_state(struct device *dev)
> > > +/** __device_links_queue_sync_state - Queue a device for sync_state() callback
> >
> > This should be
> >
> > /**
> >  * __device_links_queue_sync_state - Queue a device for sync_state() callback
> >
> > > + * @dev: Device to call sync_state() on
> > > + *
> > > + * Queues a device for a sync_state() callback when the device links write lock
> > > + * isn't held. This allows the sync_state() execution flow to use device links
> > > + * APIs.  The caller must ensure this function is called with
> > > + * device_links_write_lock() held.
> > > + *
> > > + * This function does a get_device() to make sure the device is not freed while
> > > + * on this list.
> > > + *
> > > + * So the caller must also ensure that device_links_flush_sync_list() is called
> > > + * as soon as the caller releases device_links_write_lock().  This is necessary
> > > + * to make sure the sync_state() is called in a timely fashion and the
> > > + * put_device() is called on this device.
> > > + */
> > > +static void __device_links_queue_sync_state(struct device *dev)
> >
> > Pass a list head as a second arg.
> >
> > >  {
> > >         struct device_link *link;
> > >
> > > @@ -709,12 +727,48 @@ static void __device_links_supplier_sync_state(struct device *dev)
> > >                         return;
> > >         }
> > >
> > > -       if (dev->bus->sync_state)
> > > -               dev->bus->sync_state(dev);
> > > -       else if (dev->driver && dev->driver->sync_state)
> > > -               dev->driver->sync_state(dev);
> > > -
> > > +       /*
> > > +        * Set the flag here to avoid adding the same device to the sync_list
> > > +        * more than once. This can happen if new consumers get added to the
> > > +        * device before the sync_list is flushed.
> > > +        */
> > >         dev->state_synced = true;
> > > +
> > > +       mutex_lock(&sync_lock);
> > > +
> > > +       if (list_empty(&dev->links.defer_sync)) {
> > > +               get_device(dev);
> > > +               list_add_tail(&dev->links.defer_sync, &sync_list);
> >
> > Add it to the list that you have passed as the second arg.  No locking.
> >
> > > +       } else {
> > > +               WARN_ON(1);
> > > +       }
> > > +
> > > +       mutex_unlock(&sync_lock);
> > > +}
> > > +
> > > +/** device_links_flush_sync_list - Call sync_state() on devices queued for it
> > > + *
> > > + * Calls sync_state() on all the devices that have been queued for it. This
> > > + * function is used in conjunction with __device_links_queue_sync_state().
> > > + */
> > > +static void device_links_flush_sync_list(void)
> >
> > Make it take a list to flush as an arg.
> >
> > > +{
> > > +       struct device *dev, *tmp;
> > > +
> > > +       mutex_lock(&sync_lock);
> >
> > This isn't necessary.
> >
> > > +
> > > +       list_for_each_entry_safe(dev, tmp, &sync_list, links.defer_sync) {
> > > +               list_del_init(&dev->links.defer_sync);
> > > +               device_lock(dev);
> >
> > Empty lines around this?
> >
> > > +               if (dev->bus->sync_state)
> > > +                       dev->bus->sync_state(dev);
> > > +               else if (dev->driver && dev->driver->sync_state)
> > > +                       dev->driver->sync_state(dev);
> > > +               device_unlock(dev);
> >
> > And this?
> >
> > > +               put_device(dev);
> > > +       }
> > > +
> > > +       mutex_unlock(&sync_lock);
> >
> > This isn't necessary.
> >
> > >  }
> > >
> > >  void device_links_supplier_sync_state_pause(void)
> > > @@ -738,11 +792,16 @@ void device_links_supplier_sync_state_resume(void)
> > >                 goto out;
> > >
> > >         list_for_each_entry_safe(dev, tmp, &deferred_sync, links.defer_sync) {
> > > -               __device_links_supplier_sync_state(dev);
> > > +               /*
> > > +                * Delete from deferred_sync list before queuing it to
> > > +                * sync_list because defer_sync is used for both lists.
> > > +                */
> > >                 list_del_init(&dev->links.defer_sync);
> > > +               __device_links_queue_sync_state(dev);
> >
> > Use a local list (initially empty) in this function and pass it to the above.
> >
> > >         }
> > >  out:
> > >         device_links_write_unlock();
> > > +       device_links_flush_sync_list();
> >
> > Pass the local list here too.
> >
> > >  }
> > >
> > >  static int sync_state_resume_initcall(void)
> > > @@ -815,12 +874,13 @@ void device_links_driver_bound(struct device *dev)
> > >                 if (defer_sync_state_count)
> > >                         __device_links_supplier_defer_sync(link->supplier);
> > >                 else
> > > -                       __device_links_supplier_sync_state(link->supplier);
> > > +                       __device_links_queue_sync_state(link->supplier);
> >
> > Like in the previous case, use a local list (initially empty) in this
> > function and pass it to the above.
> >
> > >         }
> > >
> > >         dev->links.status = DL_DEV_DRIVER_BOUND;
> > >
> > >         device_links_write_unlock();
> > > +       device_links_flush_sync_list();
> >
> > And pass that list here too.
> >
> > >  }
> > >
> > >  static void device_link_drop_managed(struct device_link *link)
> > > --
>
> Ok, I'll move it to a local list, add the blank lines and send out a patch?

Works for me. :-)