Re: [PATCH RESEND RFC] driver: core: don't queue device links removal for dt overlays

[Nuno Sá]

On Wed, 2024-01-31 at 14:30 +0100, Rafael J. Wysocki wrote:
> On Wed, Jan 31, 2024 at 1:20 PM Nuno Sá <noname.nuno@xxxxxxxxx> wrote:
> > 
> > On Tue, 2024-01-23 at 16:40 +0100, Nuno Sa via B4 Relay wrote:
> > > From: Nuno Sa <nuno.sa@xxxxxxxxxx>
> > > 
> > > For device links, releasing the supplier/consumer devices references
> > > happens asynchronously in device_link_release_fn(). Hence, the possible
> > > release of an of_node is also asynchronous. If these nodes were added
> > > through overlays we have a problem because this does not respect the
> > > devicetree overlays assumptions that when a changeset is
> > > being removed in __of_changeset_entry_destroy(), it must hold the last
> > > reference to that node. Due to the async nature of device links that
> > > cannot be guaranteed.
> > > 
> > > Given the above, in case one of the link consumer/supplier is part of
> > > an overlay node we call directly device_link_release_fn() instead of
> > > queueing it. Yes, it might take some significant time for
> > > device_link_release_fn() to complete because of synchronize_srcu() but
> > > we would need to, anyways, wait for all OF references to be released if
> > > we want to respect overlays assumptions.
> > > 
> > > Signed-off-by: Nuno Sa <nuno.sa@xxxxxxxxxx>
> > > ---
> > > This RFC is a follow up of a previous one that I sent to the devicetree
> > > folks [1]. It got rejected because it was not really fixing the root
> > > cause of the issue (which I do agree). Please see the link where I
> > > fully explain what the issue is.
> > > 
> > > I did also some git blaming and did saw that commit
> > > 80dd33cf72d1 ("drivers: base: Fix device link removal") introduced
> > > queue_work() as we could be releasing the last device reference and hence
> > > sleeping which is against SRCU callback requirements. However, that same
> > > commit is now making use of synchronize_srcu() which may take
> > > significant time (and I think that's the reason for the work item?).
> > > 
> > > However, given the dt overlays requirements, I'm not seeing any
> > > reason to not be able to run device_link_release_fn() synchronously if we
> > > detect an OVERLAY node is being released. I mean, even if we come up
> > > (and I did some experiments in this regard) with some async mechanism to
> > > release the OF nodes refcounts, we still need a synchronization point
> > > somewhere.
> > > 
> > > Anyways, I would like to have some feedback on how acceptable would this
> > > be or what else could I do so we can have a "clean" dt overlay removal.
> > > 
> > > I'm also including dt folks so they can give some comments on the new
> > > device_node_overlay_removal() function. My goal is to try to detect when
> > > an
> > > overlay is being removed (maybe we could even have an explicit flag for
> > > it?) and only directly call device_link_release_fn() in that case.
> > > 
> > > [1]:
> > > https://lore.kernel.org/linux-devicetree/20230511151047.1779841-1-nuno.sa@xxxxxxxxxx/
> > > ---
> > >  drivers/base/core.c | 25 ++++++++++++++++++++++++-
> > >  1 file changed, 24 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 14d46af40f9a..31ea001f6142 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -497,6 +497,18 @@ static struct attribute *devlink_attrs[] = {
> > >  };
> > >  ATTRIBUTE_GROUPS(devlink);
> > > 
> > > +static bool device_node_overlay_removal(struct device *dev)
> > > +{
> > > +     if (!dev_of_node(dev))
> > > +             return false;
> > > +     if (!of_node_check_flag(dev->of_node, OF_DETACHED))
> > > +             return false;
> > > +     if (!of_node_check_flag(dev->of_node, OF_OVERLAY))
> > > +             return false;
> > > +
> > > +     return true;
> > > +}
> > > +
> > >  static void device_link_release_fn(struct work_struct *work)
> > >  {
> > >       struct device_link *link = container_of(work, struct device_link,
> > > rm_work);
> > > @@ -532,8 +544,19 @@ static void devlink_dev_release(struct device *dev)
> > >        * 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.
> > > +      *
> > > +      * However, if any of the supplier, consumer nodes is being removed
> > > +      * through overlay removal, the expectation in
> > > +      * __of_changeset_entry_destroy() is for the node 'kref' to be 1
> > > which
> > > +      * cannot be guaranteed with the async nature of
> > > +      * device_link_release_fn(). Hence, do it synchronously for the
> > > overlay
> > > +      * case.
> > >        */
> > > -     queue_work(system_long_wq, &link->rm_work);
> > > +     if (device_node_overlay_removal(link->consumer) ||
> > > +         device_node_overlay_removal(link->supplier))
> > > +             device_link_release_fn(&link->rm_work);
> > > +     else
> > > +             queue_work(system_long_wq, &link->rm_work);
> > >  }
> > > 
> > >  static struct class devlink_class = {
> > > 
> > > ---
> > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > > change-id: 20240123-fix-device-links-overlays-5422e033a09b
> > > --
> > > 
> > > Thanks!
> > > - Nuno Sá
> > > 
> > 
> > Hi Rafael,
> > 
> > Would be nice to have your feedback on this one or if this is a complete
> > nack...
> > I think calling device_link_release_fn() synchronously is ok but I might be
> > completely wrong.
> 
> Well, it sounds like you are expecting me to confirm that what you are
> doing makes sense, but I cannot do that, because I am not sufficiently
> familiar with DT overlays.
> 

I'm trying to understand if there's no hidden issue by calling it synchronously.
(don't think there is but this is rather core stuff :)).

>From the DT guys, it would be helpful to get feedback on the new
device_node_overlay_removal() helper I'm introducing. The goal is to just do the
sync release in case we detect a node being removed as a result of an overlay
removal.

> You first need to convince yourself that you are not completely wrong.

I mean, the problem is definitely real and if you see the link I pasted in the
cover, this will all lead to big splats.

> 
> > +Cc Saravan as he should also be very familiar with device_links and see if
> > the
> > above fairly simple solution is sane.
> > 
> > I also don't want to be pushy as I know you guys are all very busy but it's
> > (i
> > think) the third time I resend the patch :)
> 
> Sorry about that, I haven't realized that my input is requisite.
> 

Yeah, get_mantainers gives me you and Greg but I think you're the main dev on
dev_links right?

> So the patch not only calls device_link_release_fn() synchronously,
> but it also calls this function directly and I, personally, wouldn't
> do at least the latter.
> 

So you mean adding something like adding a new

device_link_release(struct device_link *link) helper
and either call it synchronously from devlink_dev_release() or asynchronously
from device_link_release_fn()?

I can drop the RFC and send a patch with the above...

- Nuno Sá