Re: [PATCH v3] driver core: Fix SYNC_STATE_ONLY device link implementation

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

On Tue, May 19, 2020 at 8:30 AM Saravana Kannan <saravanak@google.com> wrote:
>
> When SYNC_STATE_ONLY support was added in commit 05ef983e0d65 ("driver
> core: Add device link support for SYNC_STATE_ONLY flag"),
> device_link_add() incorrectly skipped adding the new SYNC_STATE_ONLY
> device link to the supplier's and consumer's "device link" list.
>
> This causes multiple issues:
> - The device link is lost forever from driver core if the caller
>   didn't keep track of it (caller typically isn't expected to). This is
>   a memory leak.
> - The device link is also never visible to any other code path after
>   device_link_add() returns.
>
> If we fix the "device link" list handling, that exposes a bunch of
> issues.
>
> 1. The device link "status" state management code rightfully doesn't
> handle the case where a DL_FLAG_MANAGED device link exists between a
> supplier and consumer, but the consumer manages to probe successfully
> before the supplier. The addition of DL_FLAG_SYNC_STATE_ONLY links break
> this assumption. This causes device_links_driver_bound() to throw a
> warning when this happens.
>
> Since DL_FLAG_SYNC_STATE_ONLY device links are mainly used for creating
> proxy device links for child device dependencies and aren't useful once
> the consumer device probes successfully, this patch just deletes
> DL_FLAG_SYNC_STATE_ONLY device links once its consumer device probes.
> This way, we avoid the warning, free up some memory and avoid
> complicating the device links "status" state management code.
>
> 2. Creating a DL_FLAG_STATELESS device link between two devices that
> already have a DL_FLAG_SYNC_STATE_ONLY device link will result in the
> DL_FLAG_STATELESS flag not getting set correctly. This patch also fixes
> this.
>
> Lastly, this patch also fixes minor whitespace issues.
>
> Cc: stable@vger.kernel.org
> Fixes: 05ef983e0d65 ("driver core: Add device link support for SYNC_STATE_ONLY flag")
> Signed-off-by: Saravana Kannan <saravanak@google.com>

No issues found, so

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
> v3:
> - Added this changelog text
> v2:
> - Delete DL_FLAG_SYNC_STATE_ONLY device links on consumer probe
> - Set DL_FLAG_STATELESS correct when added to an existing
>   DL_FLAG_SYNC_STATE_ONLY device link.
> v1:
> - Add device link to list
> - Minor whitespace fixes
>
>  drivers/base/core.c | 61 +++++++++++++++++++++++++++++----------------
>  1 file changed, 39 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 84c569726d75..f804e561e0a2 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -363,13 +363,12 @@ struct device_link *device_link_add(struct device *consumer,
>
>                 if (flags & DL_FLAG_STATELESS) {
>                         kref_get(&link->kref);
> +                       link->flags |= DL_FLAG_STATELESS;
>                         if (link->flags & DL_FLAG_SYNC_STATE_ONLY &&
> -                           !(link->flags & DL_FLAG_STATELESS)) {
> -                               link->flags |= DL_FLAG_STATELESS;
> +                           !(link->flags & DL_FLAG_STATELESS))
>                                 goto reorder;
> -                       } else {
> +                       else
>                                 goto out;
> -                       }
>                 }
>
>                 /*
> @@ -436,12 +435,16 @@ struct device_link *device_link_add(struct device *consumer,
>             flags & DL_FLAG_PM_RUNTIME)
>                 pm_runtime_resume(supplier);
>
> +       list_add_tail_rcu(&link->s_node, &supplier->links.consumers);
> +       list_add_tail_rcu(&link->c_node, &consumer->links.suppliers);
> +
>         if (flags & DL_FLAG_SYNC_STATE_ONLY) {
>                 dev_dbg(consumer,
>                         "Linked as a sync state only consumer to %s\n",
>                         dev_name(supplier));
>                 goto out;
>         }
> +
>  reorder:
>         /*
>          * Move the consumer and all of the devices depending on it to the end
> @@ -452,12 +455,9 @@ struct device_link *device_link_add(struct device *consumer,
>          */
>         device_reorder_to_tail(consumer, NULL);
>
> -       list_add_tail_rcu(&link->s_node, &supplier->links.consumers);
> -       list_add_tail_rcu(&link->c_node, &consumer->links.suppliers);
> -
>         dev_dbg(consumer, "Linked as a consumer to %s\n", dev_name(supplier));
>
> - out:
> +out:
>         device_pm_unlock();
>         device_links_write_unlock();
>
> @@ -832,6 +832,13 @@ static void __device_links_supplier_defer_sync(struct device *sup)
>                 list_add_tail(&sup->links.defer_sync, &deferred_sync);
>  }
>
> +static void device_link_drop_managed(struct device_link *link)
> +{
> +       link->flags &= ~DL_FLAG_MANAGED;
> +       WRITE_ONCE(link->status, DL_STATE_NONE);
> +       kref_put(&link->kref, __device_link_del);
> +}
> +
>  /**
>   * device_links_driver_bound - Update device links after probing its driver.
>   * @dev: Device to update the links for.
> @@ -845,7 +852,7 @@ static void __device_links_supplier_defer_sync(struct device *sup)
>   */
>  void device_links_driver_bound(struct device *dev)
>  {
> -       struct device_link *link;
> +       struct device_link *link, *ln;
>         LIST_HEAD(sync_list);
>
>         /*
> @@ -885,18 +892,35 @@ void device_links_driver_bound(struct device *dev)
>         else
>                 __device_links_queue_sync_state(dev, &sync_list);
>
> -       list_for_each_entry(link, &dev->links.suppliers, c_node) {
> +       list_for_each_entry_safe(link, ln, &dev->links.suppliers, c_node) {
> +               struct device *supplier;
> +
>                 if (!(link->flags & DL_FLAG_MANAGED))
>                         continue;
>
> -               WARN_ON(link->status != DL_STATE_CONSUMER_PROBE);
> -               WRITE_ONCE(link->status, DL_STATE_ACTIVE);
> +               supplier = link->supplier;
> +               if (link->flags & DL_FLAG_SYNC_STATE_ONLY) {
> +                       /*
> +                        * When DL_FLAG_SYNC_STATE_ONLY is set, it means no
> +                        * other DL_MANAGED_LINK_FLAGS have been set. So, it's
> +                        * save to drop the managed link completely.
> +                        */
> +                       device_link_drop_managed(link);
> +               } else {
> +                       WARN_ON(link->status != DL_STATE_CONSUMER_PROBE);
> +                       WRITE_ONCE(link->status, DL_STATE_ACTIVE);
> +               }
>
> +               /*
> +                * This needs to be done even for the deleted
> +                * DL_FLAG_SYNC_STATE_ONLY device link in case it was the last
> +                * device link that was preventing the supplier from getting a
> +                * sync_state() call.
> +                */
>                 if (defer_sync_state_count)
> -                       __device_links_supplier_defer_sync(link->supplier);
> +                       __device_links_supplier_defer_sync(supplier);
>                 else
> -                       __device_links_queue_sync_state(link->supplier,
> -                                                       &sync_list);
> +                       __device_links_queue_sync_state(supplier, &sync_list);
>         }
>
>         dev->links.status = DL_DEV_DRIVER_BOUND;
> @@ -906,13 +930,6 @@ void device_links_driver_bound(struct device *dev)
>         device_links_flush_sync_list(&sync_list, dev);
>  }
>
> -static void device_link_drop_managed(struct device_link *link)
> -{
> -       link->flags &= ~DL_FLAG_MANAGED;
> -       WRITE_ONCE(link->status, DL_STATE_NONE);
> -       kref_put(&link->kref, __device_link_del);
> -}
> -
>  /**
>   * __device_links_no_driver - Update links of a device without a driver.
>   * @dev: Device without a drvier.
> --
> 2.26.2.761.g0e0b3e54be-goog
>