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Message-ID: <c2be9a4cbbe6946e2f98a69d915c22bc52c9f1d9.camel@gmail.com>
Subject: Re: [PATCH v3 2/2] of: overlay: Synchronize of_overlay_remove()
 with the devlink removals
From: Nuno =?ISO-8859-1?Q?S=E1?= <noname.nuno@gmail.com>
To: Herve Codina <herve.codina@bootlin.com>, Saravana Kannan
	 <saravanak@google.com>, Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "Rafael J. Wysocki"
 <rafael@kernel.org>, Frank Rowand <frowand.list@gmail.com>, Lizhi Hou
 <lizhi.hou@amd.com>, Max Zhen <max.zhen@amd.com>, Sonal Santan
 <sonal.santan@amd.com>, Stefano Stabellini <stefano.stabellini@xilinx.com>,
  Jonathan Cameron <Jonathan.Cameron@huawei.com>,
 linux-kernel@vger.kernel.org, devicetree@vger.kernel.org, Allan Nielsen
 <allan.nielsen@microchip.com>, Horatiu Vultur
 <horatiu.vultur@microchip.com>, Steen Hegelund
 <steen.hegelund@microchip.com>, Luca Ceresoli <luca.ceresoli@bootlin.com>,
 Nuno Sa <nuno.sa@analog.com>, Thomas Petazzoni
 <thomas.petazzoni@bootlin.com>,  stable@vger.kernel.org
Date: Tue, 05 Mar 2024 11:47:22 +0100
In-Reply-To: <20240305112708.56869e4c@bootlin.com>
References: <20240229105204.720717-1-herve.codina@bootlin.com>
	 <20240229105204.720717-3-herve.codina@bootlin.com>
	 <acb69aa8c1a4c4e9849123ef538b9646a71507a0.camel@gmail.com>
	 <20240304152202.GA222088-robh@kernel.org>
	 <20240304174933.7ad023f9@bootlin.com>
	 <CAGETcx-tmyJA30GtdU_dO9tWFoK+rO5tm-On4tPR7oQotnMkqQ@mail.gmail.com>
	 <2f497783da939f13d8c8faeab931cac0ef9c98eb.camel@gmail.com>
	 <20240305112708.56869e4c@bootlin.com>
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On Tue, 2024-03-05 at 11:27 +0100, Herve Codina wrote:
> Hi Nuno, Saravana, Rob,
>=20
> On Tue, 05 Mar 2024 08:36:45 +0100
> Nuno S=C3=A1 <noname.nuno@gmail.com> wrote:
>=20
> > On Mon, 2024-03-04 at 22:47 -0800, Saravana Kannan wrote:
> > > On Mon, Mar 4, 2024 at 8:49=E2=80=AFAM Herve Codina <herve.codina@boo=
tlin.com>
> > > wrote:=C2=A0=20
> > > >=20
> > > > Hi Rob,
> > > >=20
> > > > On Mon, 4 Mar 2024 09:22:02 -0600
> > > > Rob Herring <robh@kernel.org> wrote:
> > > >=20
> > > > ...
> > > > =C2=A0=20
> > > > > > > @@ -853,6 +854,14 @@ static void free_overlay_changeset(struc=
t
> > > > > > > overlay_changeset *ovcs)
> > > > > > > =C2=A0{
> > > > > > > =C2=A0 int i;
> > > > > > >=20
> > > > > > > + /*
> > > > > > > +=C2=A0 * Wait for any ongoing device link removals before re=
moving
> > > > > > > some of
> > > > > > > +=C2=A0 * nodes. Drop the global lock while waiting
> > > > > > > +=C2=A0 */
> > > > > > > + mutex_unlock(&of_mutex);
> > > > > > > + device_link_wait_removal();
> > > > > > > + mutex_lock(&of_mutex);=C2=A0=20
> > > > > >=20
> > > > > > 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?=C2=A0=20
> > > > >=20
> > > > > It is also just ugly because it's the callers of
> > > > > free_overlay_changeset() that hold the lock and now we're releasi=
ng it
> > > > > behind their back.
> > > > >=20
> > > > > 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 i=
f
> > > > > applying the overlay fails?=C2=A0=20
> > >=20
> > > Rob,
> > >=20
> > > 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.
> > >=20
> > > [1] - https://lore.kernel.org/all/20231220181627.341e8789@booty/
> > > =C2=A0=20
> >=20
> > I'm still not totally convinced about that. Why not putting the check r=
ight
> > before checking the kref in __of_changeset_entry_destroy(). I'll contra=
dict
> > myself a bit because this is just theory but if we look at pci_stop_dev=
(),
> > which
> > AFAIU, could be reached from a sysfs write(), we have:
> >=20
> > device_release_driver(&dev->dev);
> > ...
> > of_pci_remove_node(dev);
> > 	of_changeset_revert(np->data);
> > 	of_changeset_destroy(np->data);
> >=20
> > So looking at the above we would hit the same issue if we flush the que=
ue in
> > free_overlay_changeset() - as the queue won't be flushed at all and we =
could
> > have devlink removal due to device_release_driver(). Right?
> >=20
> > Again, completely theoretical but seems like a reasonable one plus I'm =
not
> > understanding the push against having the flush in
> > __of_changeset_entry_destroy(). Conceptually, it looks the best place t=
o me
> > but
> > I may be missing some issue in doing it there?
>=20
> Instead of having the wait called in __of_changeset_entry_destroy() and s=
o
> called in a loop. I could move this call in the __of_changeset_entry_dest=
roy()
> caller (without any of_mutex lock drop).
>=20

Oh, good catch! At this point all the devlinks removals (related to the
changeset) should have been queued so yes, we should only need to flush onc=
e.
=20
> So this will look like this:
> --- 8< ---
> void of_changeset_destroy(struct of_changeset *ocs)
> {
> 	struct of_changeset_entry *ce, *cen;
>=20
> 	device_link_wait_removal();
>=20
> 	list_for_each_entry_safe_reverse(ce, cen, &ocs->entries, node)
> 		__of_changeset_entry_destroy(ce);
> }
> --- 8< ---
>=20
> I already tested on my system and it works correctly with
> device_link_wait_removal() only called from of_changeset_destroy()
> as proposed.
>=20
> Saravana, Nuno, Rob does it seems ok for you ?
>=20

It looks good to me...

- Nuno S=C3=A1
>=20