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SubjectRe: [PATCH 2/2] panic: Enable to print out all printk msg in buffer
Hi Petr,

On Tue, Apr 09, 2019 at 04:14:30PM +0200, Petr Mladek wrote:
> On Mon 2019-04-01 18:48:04, Feng Tang wrote:
> > Currently on panic, kernel will lower the loglevel and print out
> > new printk msg only. With this patch, user can configure the
> > "panic_print" to see all dmesg in buffer, some of which they may
> > have never seen due to the loglevel setting.
> >
> > Signed-off-by: Feng Tang <feng.tang@intel.com>
> > ---
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index fb77e01..afe023e 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -51,6 +51,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
> > #define PANIC_PRINT_TIMER_INFO 0x00000004
> > #define PANIC_PRINT_LOCK_INFO 0x00000008
> > #define PANIC_PRINT_FTRACE_INFO 0x00000010
> > +#define PANIC_PRINT_ALL_PRINTK_MSG 0x00000020
> > unsigned long panic_print;
> >
> > ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
> > @@ -134,6 +135,13 @@ EXPORT_SYMBOL(nmi_panic);
> >
> > static void panic_print_sys_info(void)
> > {
> > + bool flush_all_dmesg = false;
> > +
> > + if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> > + flush_all_dmesg = true;
> > +
> > + console_flush_on_panic(flush_all_dmesg);
>
> We should:
>
> + Flush the latest messages before we replay the log.
>
> + Show some header line before replaing the log.
>
>
> Therefore, I would keep console_flush_on_panic() as is.
> Then I would add:
>
> int console_replay;
>
> void console_replay_on_panic(void)
> {
> /*
> * If someone else is holding the console lock, trylock will fail
> * and may_schedule may be set. Ignore and proceed to unlock so
> * that messages are flushed out. As this can be called from any
> * context and we don't want to get preempted while flushing,
> * ensure may_schedule is cleared.
> */
> console_trylock();
> console_may_schedule = 0;
> console_replay = 1;
> console_unlock();
> }

Many thanks for the review and detailed sample codes.

Your code of moving the console_seq and console_index changing
code into concole_unlock() is much safer, which is protected by
the locl.

Initially I thought of adding a new function similar to this, but
was afraid that it is too similar to the console_flush_on_panic() :)

>
> Then I would update cosole_unlock() with something like:
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 02ca827b8fac..14ef4e2431e7 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2386,21 +2386,32 @@ void console_unlock(void)
>
> for (;;) {
> struct printk_log *msg;
> + int reset_idx = 0;
> size_t ext_len = 0;
> - size_t len;
> + size_t len = 0;
>
> printk_safe_enter_irqsave(flags);
> raw_spin_lock(&logbuf_lock);
> +
> if (console_seq < log_first_seq) {
> len = sprintf(text,
> "** %llu printk messages dropped **\n",
> log_first_seq - console_seq);
>
> /* messages are gone, move to first one */
> + reset_idx = 1;
> + }
> +
> + if (console_replay) {
> + len += sprintf(text + len,
> + "Replaying the entire log:\n");
> + reset_idx = 1;
> + console_replay = 0;
> + }
> +
> + if (reset_idx) {
> console_seq = log_first_seq;
> console_idx = log_first_idx;
> - } else {
> - len = 0;
> }
> skip:
> if (console_seq == log_next_seq)
>
>
> Finally, it can get called from panic_print_sys_info(void)
> the following way:
>
> /* Replay existing messages before adding other sys info. */
> if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> console_replay_on_panic();

Yes, it should be the first inside panic_print_sys_info().

Thanks,
Feng

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