serial: Reduce spinlocked portion of uart_rs485_config() #173
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Commit 44b27ae ("serial: core, 8250: set RS485 termination GPIO in serial core") enabled support for RS485 termination GPIOs behind i2c expanders by setting the GPIO outside of the critical section protected by the port spinlock. Access to the i2c expander may sleep, which caused a splat with the port spinlock held. Commit 7c7f9bc ("serial: Deassert Transmit Enable on probe in driver-specific way") erroneously regressed that by spinlocking the GPIO manipulation again. Fix by moving uart_rs485_config() (the function manipulating the GPIO) outside of the spinlocked section and acquiring the spinlock inside of uart_rs485_config() for the invocation of ->rs485_config() only. This gets us one step closer to pushing the spinlock down into the ->rs485_config() callbacks which actually need it. (Some callbacks do not want to be spinlocked because they perform sleepable register accesses, see e.g. sc16is7xx_config_rs485().) Stack trace for posterity: Voluntary context switch within RCU read-side critical section! WARNING: CPU: 0 PID: 56 at kernel/rcu/tree_plugin.h:318 rcu_note_context_switch Call trace: rcu_note_context_switch __schedule schedule schedule_timeout wait_for_completion_timeout bcm2835_i2c_xfer __i2c_transfer i2c_transfer i2c_transfer_buffer_flags regmap_i2c_write _regmap_raw_write_impl _regmap_bus_raw_write _regmap_write _regmap_update_bits regmap_update_bits_base pca953x_gpio_set_value gpiod_set_raw_value_commit gpiod_set_value_nocheck gpiod_set_value_cansleep uart_rs485_config uart_add_one_port pl011_register_port pl011_probe Fixes: 7c7f9bc ("serial: Deassert Transmit Enable on probe in driver-specific way") Suggested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de> Signed-off-by: Lukas Wunner <lukas@wunner.de> Cc: stable@vger.kernel.org # v6.1+ Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
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l1k
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Sep 22, 2023
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Commit 44b27ae ("serial: core, 8250: set RS485 termination GPIO in serial core") enabled support for RS485 termination GPIOs behind i2c expanders by setting the GPIO outside of the critical section protected by the port spinlock. Access to the i2c expander may sleep, which caused a splat with the port spinlock held.
Commit 7c7f9bc ("serial: Deassert Transmit Enable on probe in driver-specific way") erroneously regressed that by spinlocking the GPIO manipulation again.
Fix by moving uart_rs485_config() (the function manipulating the GPIO) outside of the spinlocked section and acquiring the spinlock inside of uart_rs485_config() for the invocation of ->rs485_config() only.
This gets us one step closer to pushing the spinlock down into the ->rs485_config() callbacks which actually need it. (Some callbacks do not want to be spinlocked because they perform sleepable register accesses, see e.g. sc16is7xx_config_rs485().)
Stack trace for posterity:
Voluntary context switch within RCU read-side critical section!
WARNING: CPU: 0 PID: 56 at kernel/rcu/tree_plugin.h:318 rcu_note_context_switch
Call trace:
rcu_note_context_switch
__schedule
schedule
schedule_timeout
wait_for_completion_timeout
bcm2835_i2c_xfer
__i2c_transfer
i2c_transfer
i2c_transfer_buffer_flags
regmap_i2c_write
_regmap_raw_write_impl
_regmap_bus_raw_write
_regmap_write
_regmap_update_bits
regmap_update_bits_base
pca953x_gpio_set_value
gpiod_set_raw_value_commit
gpiod_set_value_nocheck
gpiod_set_value_cansleep
uart_rs485_config
uart_add_one_port
pl011_register_port
pl011_probe
Fixes: 7c7f9bc ("serial: Deassert Transmit Enable on probe in driver-specific way")
Suggested-by: Lino Sanfilippo LinoSanfilippo@gmx.de
Cc: stable@vger.kernel.org # v6.1+