[PATCH] spi: spidev: fix lock inversion between spi_lock and buf_lock

[PATCH] spi: spidev: fix lock inversion between spi_lock and buf_lock

[Fabian Godehardt]

The spidev driver previously used two mutexes, spi_lock and buf_lock,
but acquired them in different orders depending on the code path:

  write()/read(): buf_lock -> spi_lock
  ioctl():       spi_lock -> buf_lock

This AB-BA locking pattern triggers lockdep warnings and can
cause real deadlocks:

  WARNING: possible circular locking dependency detected
  spidev_ioctl() -> mutex_lock(&spidev->buf_lock)
  spidev_sync_write() -> mutex_lock(&spidev->spi_lock)
  *** DEADLOCK ***

The issue is reproducible with a simple userspace program that
performs write() and SPI_IOC_WR_MAX_SPEED_HZ ioctl() calls from
separate threads on the same spidev file descriptor.

Fix this by simplifying the locking model and removing the lock
inversion entirely. spidev_sync() no longer performs any locking,
and all callers serialize access using spi_lock.

buf_lock is removed since its functionality is fully covered by
spi_lock, eliminating the possibility of lock ordering issues.

This removes the lock inversion and prevents deadlocks without
changing userspace ABI or behaviour.

Signed-off-by: Fabian Godehardt <fg@emlix.com>
---
 drivers/spi/spidev.c | 63 ++++++++++++++++----------------------------
 1 file changed, 22 insertions(+), 41 deletions(-)

diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index 9a0160f6dc3d..f28528ed1c24 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -74,7 +74,6 @@ struct spidev_data {
 	struct list_head	device_entry;
 
 	/* TX/RX buffers are NULL unless this device is open (users > 0) */
-	struct mutex		buf_lock;
 	unsigned		users;
 	u8			*tx_buffer;
 	u8			*rx_buffer;
@@ -102,24 +101,6 @@ spidev_sync_unlocked(struct spi_device *spi, struct spi_message *message)
 	return status;
 }
 
-static ssize_t
-spidev_sync(struct spidev_data *spidev, struct spi_message *message)
-{
-	ssize_t status;
-	struct spi_device *spi;
-
-	mutex_lock(&spidev->spi_lock);
-	spi = spidev->spi;
-
-	if (spi == NULL)
-		status = -ESHUTDOWN;
-	else
-		status = spidev_sync_unlocked(spi, message);
-
-	mutex_unlock(&spidev->spi_lock);
-	return status;
-}
-
 static inline ssize_t
 spidev_sync_write(struct spidev_data *spidev, size_t len)
 {
@@ -132,7 +113,8 @@ spidev_sync_write(struct spidev_data *spidev, size_t len)
 
 	spi_message_init(&m);
 	spi_message_add_tail(&t, &m);
-	return spidev_sync(spidev, &m);
+
+	return spidev_sync_unlocked(spidev->spi, &m);
 }
 
 static inline ssize_t
@@ -147,7 +129,8 @@ spidev_sync_read(struct spidev_data *spidev, size_t len)
 
 	spi_message_init(&m);
 	spi_message_add_tail(&t, &m);
-	return spidev_sync(spidev, &m);
+
+	return spidev_sync_unlocked(spidev->spi, &m);
 }
 
 /*-------------------------------------------------------------------------*/
@@ -157,7 +140,7 @@ static ssize_t
 spidev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
 {
 	struct spidev_data	*spidev;
-	ssize_t			status;
+	ssize_t			status = -ESHUTDOWN;
 
 	/* chipselect only toggles at start or end of operation */
 	if (count > bufsiz)
@@ -165,7 +148,11 @@ spidev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
 
 	spidev = filp->private_data;
 
-	mutex_lock(&spidev->buf_lock);
+	mutex_lock(&spidev->spi_lock);
+
+	if (spidev->spi == NULL)
+		goto err_spi_removed;
+
 	status = spidev_sync_read(spidev, count);
 	if (status > 0) {
 		unsigned long	missing;
@@ -176,7 +163,9 @@ spidev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
 		else
 			status = status - missing;
 	}
-	mutex_unlock(&spidev->buf_lock);
+
+err_spi_removed:
+	mutex_unlock(&spidev->spi_lock);
 
 	return status;
 }
@@ -187,7 +176,7 @@ spidev_write(struct file *filp, const char __user *buf,
 		size_t count, loff_t *f_pos)
 {
 	struct spidev_data	*spidev;
-	ssize_t			status;
+	ssize_t			status = -ESHUTDOWN;
 	unsigned long		missing;
 
 	/* chipselect only toggles at start or end of operation */
@@ -196,13 +185,19 @@ spidev_write(struct file *filp, const char __user *buf,
 
 	spidev = filp->private_data;
 
-	mutex_lock(&spidev->buf_lock);
+	mutex_lock(&spidev->spi_lock);
+
+	if (spidev->spi == NULL)
+		goto err_spi_removed;
+
 	missing = copy_from_user(spidev->tx_buffer, buf, count);
 	if (missing == 0)
 		status = spidev_sync_write(spidev, count);
 	else
 		status = -EFAULT;
-	mutex_unlock(&spidev->buf_lock);
+
+err_spi_removed:
+	mutex_unlock(&spidev->spi_lock);
 
 	return status;
 }
@@ -379,14 +374,6 @@ spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 
 	ctlr = spi->controller;
 
-	/* use the buffer lock here for triple duty:
-	 *  - prevent I/O (from us) so calling spi_setup() is safe;
-	 *  - prevent concurrent SPI_IOC_WR_* from morphing
-	 *    data fields while SPI_IOC_RD_* reads them;
-	 *  - SPI_IOC_MESSAGE needs the buffer locked "normally".
-	 */
-	mutex_lock(&spidev->buf_lock);
-
 	switch (cmd) {
 	/* read requests */
 	case SPI_IOC_RD_MODE:
@@ -510,7 +497,6 @@ spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		break;
 	}
 
-	mutex_unlock(&spidev->buf_lock);
 	spi_dev_put(spi);
 	mutex_unlock(&spidev->spi_lock);
 	return retval;
@@ -541,9 +527,6 @@ spidev_compat_ioc_message(struct file *filp, unsigned int cmd,
 		return -ESHUTDOWN;
 	}
 
-	/* SPI_IOC_MESSAGE needs the buffer locked "normally" */
-	mutex_lock(&spidev->buf_lock);
-
 	/* Check message and copy into scratch area */
 	ioc = spidev_get_ioc_message(cmd, u_ioc, &n_ioc);
 	if (IS_ERR(ioc)) {
@@ -564,7 +547,6 @@ spidev_compat_ioc_message(struct file *filp, unsigned int cmd,
 	kfree(ioc);
 
 done:
-	mutex_unlock(&spidev->buf_lock);
 	spi_dev_put(spi);
 	mutex_unlock(&spidev->spi_lock);
 	return retval;
@@ -802,7 +784,6 @@ static int spidev_probe(struct spi_device *spi)
 	/* Initialize the driver data */
 	spidev->spi = spi;
 	mutex_init(&spidev->spi_lock);
-	mutex_init(&spidev->buf_lock);
 
 	INIT_LIST_HEAD(&spidev->device_entry);
 
-- 
2.47.3

Re: [PATCH] spi: spidev: fix lock inversion between spi_lock and buf_lock

[Mark Brown]

On Wed, 11 Feb 2026 08:26:16 +0100, Fabian Godehardt wrote:
> The spidev driver previously used two mutexes, spi_lock and buf_lock,
> but acquired them in different orders depending on the code path:
> 
>   write()/read(): buf_lock -> spi_lock
>   ioctl():       spi_lock -> buf_lock
> 
> This AB-BA locking pattern triggers lockdep warnings and can
> cause real deadlocks:
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next

Thanks!

[1/1] spi: spidev: fix lock inversion between spi_lock and buf_lock
      commit: 40534d19ed2afb880ecf202dab26a8e7a5808d16

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark