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

[Nuno Sá]

On Thu, 2024-03-07 at 13:16 +0100, Herve Codina wrote:
> Hi Nuno,
> 
> On Thu, 07 Mar 2024 12:50:52 +0100
> Nuno Sá <noname.nuno@xxxxxxxxx> wrote:
> 
> > 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/
> > 
> 
> We discussed that point and I understood that you were ok to do that on your
> side:
>  
> https://lore.kernel.org/linux-kernel/f42ceee61ddb8b50c347589649d4131476ab5d81.camel@xxxxxxxxx/
> 
> Sorry if I misunderstood.

Oh, yeah, I can do that. But given Saravana reply I thought the expectation is to
have the queue already allocated in devlink_class_init().

> 
> I am going to wait for other comments on this current series before re-sending
> with our 'Reviewed-by' removed if needed. Let me know.
> 

Anyways, if your expectation was for me to do it later, fine. No need to remove the
tag. Sorry for the noise.

- Nuno Sá
>