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SubjectRe: [PATCH 2/2] panic: Enable to print out all printk msg in buffer
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>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 1 +
> kernel/panic.c | 9 ++++++++-
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2b8ee90..7b15c94 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3135,6 +3135,7 @@
> bit 2: print timer info
> bit 3: print locks info if CONFIG_LOCKDEP is on
> bit 4: print ftrace buffer
> + bit 5: print all printk messages in buffer
>
> panic_on_warn panic() instead of WARN(). Useful to cause kdump
> on a WARN().
> 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();
}

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();


Best Regards,
Petr

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