Messages in this thread Patch in this message | | | Date | Mon, 25 Jul 2022 17:16:16 +0200 | From | Sebastian Andrzej Siewior <> | Subject | [PATCH v3] printk: Skip console drivers on PREEMPT_RT. |
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printk might be invoked in a context with disabled interrupts and or preemption and additionally disables interrupts before it invokes the console drivers. This behaviour is not desired on PREEMPT_RT: - The console driver are using spinlock_t based locking which become sleeping locks on PREEMPT_RT and must not be acquired with disabled interrupts (or preemption).
- The locks within the console drivers must remain sleeping locks and they must not disable interrupts. Printing (and polling for its completion) at 115200 baud on an UART takes too long for PREEMPT_RT in general and so raises the latency of the IRQ-off time of the system beyond acceptable levels.
Skip printing to the console as temporary workaround until the printing threads and atomic consoles have been introduced or another solution which is compatible with the PREEMPT_RT approach. With this change, the user will not see any kernel message printed to the console but can retrieve the printk buffer from userland (via the dmesg command). This allows enable PREEMPT_RT as a whole without disabling printk and loosing all kernel output.
Disable console printing on PREEMPT_RT.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- v2…v3: - Reword commit message by adding a few details/ explanations.
v1…v2: - Use __console_unlock() as suggested by John.
kernel/printk/printk.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
--- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2843,6 +2843,16 @@ void console_unlock(void) } /* + * On PREEMPT_RT it is not possible to invoke console drivers with + * disabled interrupts and or preemption. Therefore all drivers are + * skipped and the output can be retrieved from the buffer. + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) { + __console_unlock(); + return; + } + + /* * Console drivers are called with interrupts disabled, so * @console_may_schedule should be cleared before; however, we may * end up dumping a lot of lines, for example, if called from
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