Re: [PATCH] i2c: rcar: avoid race when unregistering slave

From: "Niklas Söderlund" <niklas.soderlund@ragnatech.se>
To: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: linux-i2c@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
	Ray Jui <ray.jui@broadcom.com>,
	Scott Branden <scott.branden@broadcom.com>,
	Dhananjay Phadke <dphadke@linux.microsoft.com>,
	bcm-kernel-feedback-list@broadcom.com,
	Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Subject: Re: [PATCH] i2c: rcar: avoid race when unregistering slave
Date: Mon, 27 Jul 2020 12:55:38 +0200	[thread overview]
Message-ID: <20200727105538.GI2729799@oden.dyn.berto.se> (raw)
In-Reply-To: <20200726161606.15315-1-wsa+renesas@sang-engineering.com>

Hi Wolfram,

Thanks for your patch.

On 2020-07-26 18:16:06 +0200, Wolfram Sang wrote:
> Due to the lockless design of the driver, it is theoretically possible
> to access a NULL pointer, if a slave interrupt was running while we were
> unregistering the slave. To make this rock solid, disable the interrupt
> for a short time while we are clearing the interrupt_enable register.
> This patch is purely based on code inspection. The OOPS is super-hard to
> trigger because clearing SAR (the address) makes interrupts even more
> unlikely to happen as well. While here, reinit SCR to SDBS because this
> bit should always be set according to documentation. There is no effect,
> though, because the interface is disabled.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>

> ---
> 
> Some people on CC here which encountered the same issue with the
> bcm-iproc driver. Does something like this work for you, too?
> 
>  drivers/i2c/busses/i2c-rcar.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-rcar.c b/drivers/i2c/busses/i2c-rcar.c
> index 8dd35522d95a..0f73f0681a6e 100644
> --- a/drivers/i2c/busses/i2c-rcar.c
> +++ b/drivers/i2c/busses/i2c-rcar.c
> @@ -871,12 +871,14 @@ static int rcar_unreg_slave(struct i2c_client *slave)
>  
>  	WARN_ON(!priv->slave);
>  
> -	/* disable irqs and ensure none is running before clearing ptr */
> +	/* ensure no irq is running before clearing ptr */
> +	disable_irq(priv->irq);
>  	rcar_i2c_write(priv, ICSIER, 0);
> -	rcar_i2c_write(priv, ICSCR, 0);
> +	rcar_i2c_write(priv, ICSSR, 0);
> +	enable_irq(priv->irq);
> +	rcar_i2c_write(priv, ICSCR, SDBS);
>  	rcar_i2c_write(priv, ICSAR, 0); /* Gen2: must be 0 if not using slave */
>  
> -	synchronize_irq(priv->irq);
>  	priv->slave = NULL;
>  
>  	pm_runtime_put(rcar_i2c_priv_to_dev(priv));
> -- 
> 2.20.1
> 

-- 
Regards,
Niklas Söderlund