CVE-2021-47469

In the Linux kernel, the following vulnerability has been resolved: spi:
Fix deadlock when adding SPI controllers on SPI buses Currently we have a
global spi_add_lock which we take when adding new devices so that we can
check that we’re not trying to reuse a chip select that’s already
controlled. This means that if the SPI device is itself a SPI controller
and triggers the instantiation of further SPI devices we trigger a deadlock
as we try to register and instantiate those devices while in the process of
doing so for the parent controller and hence already holding the global
spi_add_lock. Since we only care about concurrency within a single SPI bus
move the lock to be per controller, avoiding the deadlock. This can be
easily triggered in the case of spi-mux.