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SubjectRe: [RFC PATCH v1 12/25] printk: minimize console locking implementation
On Tue 2019-02-12 15:29:50, John Ogness wrote:
> Since printing of the printk buffer is now handled by the printk
> kthread, minimize the console locking functions to just handle
> locking of the console.
>
> NOTE: With this console_flush_on_panic will no longer flush.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
> kernel/printk/printk.c | 255 +------------------------------------------------
> 1 file changed, 1 insertion(+), 254 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 073ff9fd6872..ece54c24ea0d 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -209,19 +209,7 @@ static int nr_ext_console_drivers;
>
> static int __down_trylock_console_sem(unsigned long ip)
> {
> - int lock_failed;
> - unsigned long flags;
> -
> - /*
> - * Here and in __up_console_sem() we need to be in safe mode,
> - * because spindump/WARN/etc from under console ->lock will
> - * deadlock in printk()->down_trylock_console_sem() otherwise.
> - */
> - printk_safe_enter_irqsave(flags);
> - lock_failed = down_trylock(&console_sem);
> - printk_safe_exit_irqrestore(flags);
> -
> - if (lock_failed)
> + if (down_trylock(&console_sem))
> return 1;
> mutex_acquire(&console_lock_dep_map, 0, 1, ip);
> return 0;
> @@ -230,13 +218,9 @@ static int __down_trylock_console_sem(unsigned long ip)
>
> static void __up_console_sem(unsigned long ip)
> {
> - unsigned long flags;
> -
> mutex_release(&console_lock_dep_map, 1, ip);
>
> - printk_safe_enter_irqsave(flags);
> up(&console_sem);
> - printk_safe_exit_irqrestore(flags);
> }
> #define up_console_sem() __up_console_sem(_RET_IP_)
>

It might be obvious from the previous mails. But just to be sure.

I would remove printk_safe stuff in one patch after switching
to the new ring buffer implementation.


> @@ -1498,82 +1482,6 @@ static void format_text(struct printk_log *msg, u64 seq,
> }
>
> /*
> - * Special console_lock variants that help to reduce the risk of soft-lockups.
> - * They allow to pass console_lock to another printk() call using a busy wait.
> - */
[...]
> -static void console_lock_spinning_enable(void)

The console waiter logic is another story. It can get removed only
after we have a reasonable alternative. That means an acceptable
offload that handles emergency situations and sudden death
reasonable well.

I would move this into a separate patchset.

Best Regards,
Petr

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