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Subjectwake_up_all: was: Re: [PATCH printk v1 10/13] printk: add kthread console printers
On Mon 2022-02-07 20:49:20, John Ogness wrote:
> Create a kthread for each console to perform console printing. During
> normal operation (@system_state == SYSTEM_RUNNING), the kthread
> printers are responsible for all printing on their respective
> consoles.
>
> During non-normal operation, console printing is done as it has been:
> within the context of the printk caller or within irq work triggered
> by the printk caller.
>
> Console printers synchronize against each other and against console
> lockers by taking the console lock for each message that is printed.
> ---
> include/linux/console.h | 2 +
> kernel/printk/printk.c | 159 +++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 159 insertions(+), 2 deletions(-)
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3371,7 +3526,7 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
> }
>
> if (pending & PRINTK_PENDING_WAKEUP)
> - wake_up_interruptible(&log_wait);
> + wake_up_interruptible_all(&log_wait);

Good catch. I am curious how this worked so far. It looks like only
the first waiter was waken and others had to wait for another new
message.

The typical problem with wake_up_interruptible_all() is that it wakes
many processes that needs to take the same lock and block each
other. There is even a name for this situation but I do not recall it.

My understanding is that wake_up_interruptible() requires that
the waken process wakes the next waiter when done. But I do not see
it guaranteed for @log_wait wait queue.

Best Regards,
Petr

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