AZL-86157

In the Linux kernel, the following vulnerability has been resolved:

spi: spidev: fix lock inversion between spi_lock and buf_lock

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.