Re: [PATCH v5 1/2] driver core: Introduce device_link_wait_removal()

[Nuno Sá]

Hi Herve,


On Thu, 2024-03-07 at 12:10 +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.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> Tested-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> Reviewed-by: Nuno Sa <nuno.sa@xxxxxxxxxx>
> ---
>  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..48b28c59c592 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.
> +	 */
> +	flush_workqueue(device_link_wq);
> +}
> +EXPORT_SYMBOL_GPL(device_link_wait_removal);
> +
>  static struct class devlink_class = {
>  	.name = "devlink",
>  	.dev_groups = devlink_groups,
> @@ -4099,9 +4114,14 @@ int __init devices_init(void)
>  	sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
>  	if (!sysfs_dev_char_kobj)
>  		goto char_kobj_err;
> +	device_link_wq = alloc_workqueue("device_link_wq", 0, 0);

My rb tag was with the assumption this is moved into devlink_class_init(). IIUC,
Saravana also agreed with that [1]. But it looks like he missed that we are
allocating the queue in devices_init() and not in devlink_class_init().

I'm also not sure if this is in line with what Rafael wanted for ccing stable. How do
we know the next patch depends on this one?

[1]: https://lore.kernel.org/lkml/CAGETcx_gNWOTsSZMaZu+XU1-5Z60WEcMhw08t4Sn_-YgkCCUmA@xxxxxxxxxxxxxx/

- Nuno Sá