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

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Message-ID: <20210716210245.13240-1-alexandru.tachici@analog.com>
Date:   Sat, 17 Jul 2021 00:02:44 +0300
From:   <alexandru.tachici@...log.com>
To:     <linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
        <broonie@...nel.org>
CC:     <nuno.sa@...log.com>, <bootc@...tc.net>, <swarren@...dotorg.org>,
        <bcm-kernel-feedback-list@...adcom.com>, <rjui@...adcom.com>,
        <f.fainelli@...il.com>, <nsaenz@...nel.org>,
        Alexandru Tachici <alexandru.tachici@...log.com>
Subject: [PATCH 0/1] spi: spi-bcm2835: Fix deadlock

From: Alexandru Tachici <alexandru.tachici@...log.com>

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@analog.com/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(-)

--
2.25.1