Re: [PATCH -next] spi: spidev: fix a recursive locking error

[Max Krummenacher <max.krummenacher@xxxxxxxxxxx>]

Hi

On Mon, 2023-01-16 at 15:41 +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> 
> When calling spidev_message() from the one of the ioctl() callbacks, the
> spi_lock is already taken. When we then end up calling spidev_sync(), we
> get the following splat:
> 
> [  214.047619]
> [  214.049198] ============================================
> [  214.054533] WARNING: possible recursive locking detected
> [  214.059858] 6.2.0-rc3-0.0.0-devel+git.97ec4d559d93 #1 Not tainted
> [  214.065969] --------------------------------------------
> [  214.071290] spidev_test/1454 is trying to acquire lock:
> [  214.076530] c4925dbc (&spidev->spi_lock){+.+.}-{3:3}, at: spidev_ioctl+0x8e0/0xab8
> [  214.084164]
> [  214.084164] but task is already holding lock:
> [  214.090007] c4925dbc (&spidev->spi_lock){+.+.}-{3:3}, at: spidev_ioctl+0x44/0xab8
> [  214.097537]
> [  214.097537] other info that might help us debug this:
> [  214.104075]  Possible unsafe locking scenario:
> [  214.104075]
> [  214.110004]        CPU0
> [  214.112461]        ----
> [  214.114916]   lock(&spidev->spi_lock);
> [  214.118687]   lock(&spidev->spi_lock);
> [  214.122457]
> [  214.122457]  *** DEADLOCK ***
> [  214.122457]
> [  214.128386]  May be due to missing lock nesting notation
> [  214.128386]
> [  214.135183] 2 locks held by spidev_test/1454:
> [  214.139553]  #0: c4925dbc (&spidev->spi_lock){+.+.}-{3:3}, at: spidev_ioctl+0x44/0xab8
> [  214.147524]  #1: c4925e14 (&spidev->buf_lock){+.+.}-{3:3}, at: spidev_ioctl+0x70/0xab8
> [  214.155493]
> [  214.155493] stack backtrace:
> [  214.159861] CPU: 0 PID: 1454 Comm: spidev_test Not tainted 6.2.0-rc3-0.0.0-devel+git.97ec4d559d93 #1
> [  214.169012] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
> [  214.175555]  unwind_backtrace from show_stack+0x10/0x14
> [  214.180819]  show_stack from dump_stack_lvl+0x60/0x90
> [  214.185900]  dump_stack_lvl from __lock_acquire+0x874/0x2858
> [  214.191584]  __lock_acquire from lock_acquire+0xfc/0x378
> [  214.196918]  lock_acquire from __mutex_lock+0x9c/0x8a8
> [  214.202083]  __mutex_lock from mutex_lock_nested+0x1c/0x24
> [  214.207597]  mutex_lock_nested from spidev_ioctl+0x8e0/0xab8
> [  214.213284]  spidev_ioctl from sys_ioctl+0x4d0/0xe2c
> [  214.218277]  sys_ioctl from ret_fast_syscall+0x0/0x1c
> [  214.223351] Exception stack(0xe75cdfa8 to 0xe75cdff0)
> [  214.228422] dfa0:                   00000000 00001000 00000003 40206b00 bee266e8 bee266e0
> [  214.236617] dfc0: 00000000 00001000 006a71a0 00000036 004c0040 004bfd18 00000000 00000003
> [  214.244809] dfe0: 00000036 bee266c8 b6f16dc5 b6e8e5f6
> 
> Fix it by introducing an unlocked variant of spidev_sync() and calling it
> from spidev_message() while other users who don't check the spidev->spi's
> existence keep on using the locking flavor.
> 
> Reported-by: Francesco Dolcini <francesco@xxxxxxxxxx>
> Fixes: 1f4d2dd45b6e ("spi: spidev: fix a race condition when accessing spidev->spi")
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

Thanks for the quick fix.
I tested on a Colibri iMX7 which showed the bug before the patch is applied
but not after.

Tested-by: Max Krummenacher <max.krummenacher@xxxxxxxxxxx>

Regards
Max
> ---
>  drivers/spi/spidev.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
> index 8ef22ebcde1f..892965ac8fdf 100644
> --- a/drivers/spi/spidev.c
> +++ b/drivers/spi/spidev.c
> @@ -89,10 +89,22 @@ MODULE_PARM_DESC(bufsiz, "data bytes in biggest supported SPI message");
>  
>  /*-------------------------------------------------------------------------*/
>  
> +static ssize_t
> +spidev_sync_unlocked(struct spi_device *spi, struct spi_message *message)
> +{
> +	ssize_t status;
> +
> +	status = spi_sync(spi, message);
> +	if (status == 0)
> +		status = message->actual_length;
> +
> +	return status;
> +}
> +
>  static ssize_t
>  spidev_sync(struct spidev_data *spidev, struct spi_message *message)
>  {
> -	int status;
> +	ssize_t status;
>  	struct spi_device *spi;
>  
>  	mutex_lock(&spidev->spi_lock);
> @@ -101,12 +113,10 @@ spidev_sync(struct spidev_data *spidev, struct spi_message *message)
>  	if (spi == NULL)
>  		status = -ESHUTDOWN;
>  	else
> -		status = spi_sync(spi, message);
> -
> -	if (status == 0)
> -		status = message->actual_length;
> +		status = spidev_sync_unlocked(spi, message);
>  
>  	mutex_unlock(&spidev->spi_lock);
> +
>  	return status;
>  }
>  
> @@ -294,7 +304,7 @@ static int spidev_message(struct spidev_data *spidev,
>  		spi_message_add_tail(k_tmp, &msg);
>  	}
>  
> -	status = spidev_sync(spidev, &msg);
> +	status = spidev_sync_unlocked(spidev->spi, &msg);
>  	if (status < 0)
>  		goto done;
>