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Message-ID: <ed442b6916016b3a40782dc32538fc517715db6c.camel@gmail.com>
Subject: Re: [PATCH v4 1/2] driver core: Introduce device_link_wait_removal()
From: Nuno =?ISO-8859-1?Q?S=E1?= <noname.nuno@gmail.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Herve Codina <herve.codina@bootlin.com>, Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>, Rob Herring <robh+dt@kernel.org>, Frank
 Rowand <frowand.list@gmail.com>, Saravana Kannan <saravanak@google.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: Wed, 06 Mar 2024 14:05:16 +0100
In-Reply-To: <CAJZ5v0gWCo9nDAHkzeD08tTKoE0DE0ocht-Qq4zA7P59y9KeuQ@mail.gmail.com>
References: <20240306085007.169771-1-herve.codina@bootlin.com>
	 <20240306085007.169771-2-herve.codina@bootlin.com>
	 <1fff8742a13c28dd7e1dda47ad2d6fa8e21e421e.camel@gmail.com>
	 <CAJZ5v0gWCo9nDAHkzeD08tTKoE0DE0ocht-Qq4zA7P59y9KeuQ@mail.gmail.com>
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On Wed, 2024-03-06 at 13:43 +0100, Rafael J. Wysocki wrote:
> On Wed, Mar 6, 2024 at 10:17=E2=80=AFAM Nuno S=C3=A1 <noname.nuno@gmail.c=
om> wrote:
> >=20
> > On Wed, 2024-03-06 at 09:50 +0100, Herve Codina wrote:
> > > The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > > introduces a workqueue to release the consumer and supplier devices u=
sed
> > > 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.
> > >=20
> > > Nothing is present to provide some synchronisation with this workqueu=
e
> > > in order to ensure that all ongoing releasing operations are done and
> > > so, some other operations can be started safely.
> > >=20
> > > For instance, in the following sequence:
> > > =C2=A0 1) of_platform_depopulate()
> > > =C2=A0 2) of_overlay_remove()
> > >=20
> > > During the step 1, devices are released and related devlinks are remo=
ved
> > > (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 ra=
ise
> > > warnings related to missing of_node_put():
> > > =C2=A0 ERROR: memory leak, expected refcount 1 instead of 2
> > >=20
> > > Indeed, the missing of_node_put() call is going to be done, too late,
> > > from the workqueue job execution.
> > >=20
> > > 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.
> > >=20
> > > Fixes: 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> > > ---
> >=20
> > With the below addressed:
> >=20
> > Reviewed-by: Nuno Sa <nuno.sa@analog.com>
> >=20
> > > =C2=A0drivers/base/core.c=C2=A0=C2=A0=C2=A0 | 26 ++++++++++++++++++++=
+++---
> > > =C2=A0include/linux/device.h |=C2=A0 1 +
> > > =C2=A02 files changed, 24 insertions(+), 3 deletions(-)
> > >=20
> > > 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);
> > > =C2=A0static void __fw_devlink_link_to_consumers(struct device *dev);
> > > =C2=A0static bool fw_devlink_drv_reg_done;
> > > =C2=A0static bool fw_devlink_best_effort;
> > > +static struct workqueue_struct *device_link_wq;
> > >=20
> > > =C2=A0/**
> > > =C2=A0 * __fwnode_link_add - Create a link between two fwnode_handles=
.
> > > @@ -532,12 +533,26 @@ static void devlink_dev_release(struct device *=
dev)
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /*
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * It may take a while to complet=
e this work because of the SRCU
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * synchronization in device_link=
_release_fn() and if the consumer
> > > or
> > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * supplier devices get deleted when i=
t runs, so put it into the
> > > "long"
> > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * workqueue.
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * supplier devices get deleted when i=
t runs, so put it into the
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * dedicated workqueue.
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
> > > -=C2=A0=C2=A0=C2=A0=C2=A0 queue_work(system_long_wq, &link->rm_work);
> > > +=C2=A0=C2=A0=C2=A0=C2=A0 queue_work(device_link_wq, &link->rm_work);
> > > =C2=A0}
> > >=20
> > > +/**
> > > + * device_link_wait_removal - Wait for ongoing devlink removal jobs =
to
> > > terminate
> > > + */
> > > +void device_link_wait_removal(void)
> > > +{
> > > +=C2=A0=C2=A0=C2=A0=C2=A0 /*
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * devlink removal jobs are queued in =
the dedicated work queue.
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * To be sure that all removal jobs ar=
e terminated, ensure that any
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * scheduled work has run to completio=
n.
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
> > > +=C2=A0=C2=A0=C2=A0=C2=A0 flush_workqueue(device_link_wq);
> > > +}
> > > +EXPORT_SYMBOL_GPL(device_link_wait_removal);
> > > +
> > > =C2=A0static struct class devlink_class =3D {
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 .name =3D "devlink",
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 .dev_groups =3D devlink_groups,
> > > @@ -4099,9 +4114,14 @@ int __init devices_init(void)
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 sysfs_dev_char_kobj =3D kobject_create=
_and_add("char", dev_kobj);
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (!sysfs_dev_char_kobj)
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0 goto char_kobj_err;
> > > +=C2=A0=C2=A0=C2=A0=C2=A0 device_link_wq =3D alloc_workqueue("device_=
link_wq", 0, 0);
> > > +=C2=A0=C2=A0=C2=A0=C2=A0 if (!device_link_wq)
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 goto wq_err;
> > >=20
> >=20
> > I can't still agree with this. Why not doing it in devlink_class_init()=
?
> > This is
> > devlink specific so it makes complete sense to me.
>=20
> If you do that in devlink_class_init() and it fails, you essentially
> cause the creation of every device link to fail.=C2=A0 IOW, you try to li=
ve
> without device links and pretend that it is all OK.=C2=A0 That won't get
> you very far, especially on systems where DT is used.
>=20
> Doing it here, if it fails, you prevent the driver model from working
> at all (because one of its necessary components is unavailable), which
> arguably is a better choice.

That makes sense but then the only thing I still don't fully get is why we =
have
a separate devlink_class_init() initcall for registering the devlink class
(which can also fail)... What I take from the above is that we should fail =
the
driver model if one of it's fundamental components fails so I would say we
should merge devlink_class_init() with device_init() otherwise it's a bit
confusing (at least to me) and gives the idea that it's ok for the driver m=
odel
to exist without the links (unless I'm missing some other reason for the de=
vlink
init function).

- Nuno S=C3=A1