[PATCH 0/1] spi: spi-bcm2835: Fix deadlock

[<alexandru.tachici@xxxxxxxxxx>]

From: Alexandru Tachici <alexandru.tachici@xxxxxxxxxx>

The bcm2835_spi_transfer_one function can create a deadlock
if it is called while another thread already has the
CCF lock.

This behavior was observed at boot and when trying to
print the clk_summary debugfs. I had registered
at the time multiple clocks of AD9545 through the CCF.
Tested this using an RPi 4 connected to AD9545 through SPI.

See upstream attempt here:
https://lore.kernel.org/lkml/20210614070718.78041-3-alexandru.tachici@xxxxxxxxxx/T/

This can happen to any other clock that needs to read
the rate/phase from hardware using the SPI. Because
when issuing a clk_get_rate/phase, the requesting thread
already holds the CCF lock. If another thread, in this case
the one that does the spi transfer tries the same, it will cause
a deadlock. This happens by chance because not always
every spi request gets deferred to a khthread.

Alexandru Tachici (1):
  spi: spi-bcm2835: Fix deadlock

 drivers/spi/spi-bcm2835.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

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2.25.1