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Message-ID: <7eb7453244edace460f8c6fb91cb92a86fe2fc58.camel@gmail.com>
Subject: Re: [PATCH v2 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>, Greg Kroah-Hartman
	 <gregkh@linuxfoundation.org>, "Rafael J. Wysocki" <rafael@kernel.org>, Rob
	Herring <robh+dt@kernel.org>, Frank Rowand <frowand.list@gmail.com>
Cc: 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: Thu, 29 Feb 2024 10:47:29 +0100
In-Reply-To: <20240229083953.607569-3-herve.codina@bootlin.com>
References: <20240229083953.607569-1-herve.codina@bootlin.com>
	 <20240229083953.607569-3-herve.codina@bootlin.com>
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On Thu, 2024-02-29 at 09:39 +0100, Herve Codina wrote:
> In the following sequence:
> =C2=A0 1) of_platform_depopulate()
> =C2=A0 2) of_overlay_remove()
>=20
> During the step 1, devices are destroyed and devlinks are removed.
> During the step 2, OF nodes are destroyed but
> __of_changeset_entry_destroy() can raise warnings related to missing
> of_node_put():
> =C2=A0 ERROR: memory leak, expected refcount 1 instead of 2 ...
>=20
> Indeed, during the devlink removals performed at step 1, the removal
> itself releasing the device (and the attached of_node) is done by a job
> queued in a workqueue and so, it is done asynchronously with respect to
> function calls.
> When the warning is present, of_node_put() will be called but wrongly
> too late from the workqueue job.
>=20
> In order to be sure that any ongoing devlink removals are done before
> the of_node destruction, synchronize the of_overlay_remove() with the
> devlink removals.
>=20
> Fixes: 80dd33cf72d1 ("drivers: base: Fix device link removal")
> Cc: stable@vger.kernel.org
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
> =C2=A0drivers/of/overlay.c | 9 ++++++++-
> =C2=A01 file changed, 8 insertions(+), 1 deletion(-)
>=20
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 2ae7e9d24a64..99659ae9fb28 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -853,6 +853,14 @@ static void free_overlay_changeset(struct
> overlay_changeset *ovcs)
> =C2=A0{
> =C2=A0	int i;
> =C2=A0
> +	/*
> +	 * 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 e=
lse
grabs the lock while we are in device_link_wait_removal()? Can we guarantee=
 that
we can't screw things badly?

The question is, do you have a system/use case where you can really see the
deadlock happening? Until I see one, I'm very skeptical about this. And if =
we
have one, I'm not really sure this is also the right solution for it.

- Nuno S=C3=A1