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

Max Krummenacher
2023-01-16T15:30:37Z

Hi

On Mon, 2023-01-16 at 15:41 +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
>
> 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@dolcini.it>
> Fixes: 1f4d2dd45b6e ("spi: spidev: fix a race condition when accessing spidev->spi")
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

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@toradex.com>

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;
>