Re: [PATCH v3 2/2] of: overlay: Synchronize of_overlay_remove() with the devlink removals

[Saravana Kannan]

On Mon, Mar 4, 2024 at 8:49 AM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
>
> Hi Rob,
>
> On Mon, 4 Mar 2024 09:22:02 -0600
> Rob Herring <robh@xxxxxxxxxx> wrote:
>
> ...
>
> > > > @@ -853,6 +854,14 @@ static void free_overlay_changeset(struct
> > > > overlay_changeset *ovcs)
> > > >  {
> > > >   int i;
> > > >
> > > > + /*
> > > > +  * Wait for any ongoing device link removals before removing some of
> > > > +  * nodes. Drop the global lock while waiting
> > > > +  */
> > > > + mutex_unlock(&of_mutex);
> > > > + device_link_wait_removal();
> > > > + mutex_lock(&of_mutex);
> > >
> > > I'm still not convinced we need to drop the lock. What happens if someone else
> > > grabs the lock while we are in device_link_wait_removal()? Can we guarantee that
> > > we can't screw things badly?
> >
> > It is also just ugly because it's the callers of
> > free_overlay_changeset() that hold the lock and now we're releasing it
> > behind their back.
> >
> > As device_link_wait_removal() is called before we touch anything, can't
> > it be called before we take the lock? And do we need to call it if
> > applying the overlay fails?

Rob,

This[1] scenario Luca reported seems like a reason for the
device_link_wait_removal() to be where Herve put it. That example
seems reasonable.

[1] - https://lore.kernel.org/all/20231220181627.341e8789@booty/

> >
>
> Indeed, having device_link_wait_removal() is not needed when applying the
> overlay fails.
>
> I can call device_link_wait_removal() from the caller of_overlay_remove()
> but not before the lock is taken.
> We need to call it between __of_changeset_revert_notify() and
> free_overlay_changeset() and so, the lock is taken.
>
> This lead to the following sequence:
> --- 8< ---
> int of_overlay_remove(int *ovcs_id)
> {
>         ...
>         mutex_lock(&of_mutex);
>         ...
>
>         ret = __of_changeset_revert_notify(&ovcs->cset);
>         ...
>
>         ret_tmp = overlay_notify(ovcs, OF_OVERLAY_POST_REMOVE);
>         ...
>
>         mutex_unlock(&of_mutex);
>         device_link_wait_removal();
>         mutex_lock(&of_mutex);
>
>         free_overlay_changeset(ovcs);
>         ...
>         mutex_unlock(&of_mutex);
>         ...
> }
> --- 8< ---
>
> In this sequence, the question is:
> Do we need to release the mutex lock while device_link_wait_removal() is
> called ?

In general I hate these kinds of sequences that release a lock and
then grab it again quickly. It's not always a bug, but my personal
take on that is 90% of these introduce a bug.

Drop the unlock/lock and we'll deal a deadlock if we actually hit one.
I'm also fairly certain that device_link_wait_removal() can't trigger
something else that can cause an OF overlay change while we are in the
middle of one. And like Rob said, I'm not sure this unlock/lock is a
good solution for that anyway.

Please CC me on the next series. And I'm glad folks convinced you to
use flush_workqueue(). As I said in the older series, I think
drain_workqueue() will actually break device links.

-Saravana


-Saravana