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SubjectRe: [PATCH v2 3/4] printk: Avoid livelock with heavy printk during panic
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On 1/27/22 06:50, Petr Mladek wrote:
> On Wed 2022-01-26 15:02:35, Stephen Brennan wrote:
>> During panic(), if another CPU is writing heavily the kernel log (e.g.
>> via /dev/kmsg), then the panic CPU may livelock writing out its messages
>> to the console. Note when too many messages are dropped during panic and
>> suppress further printk, except from the panic CPU. This could result in
>> some important messages being dropped. However, messages are already
>> being dropped, so this approach at least prevents a livelock.
>>
>> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
>> ---
>>
>> Notes:
>> v2: Add pr_warn when we suppress printk on non-panic CPU
>>
>> kernel/printk/printk.c | 16 ++++++++++++++++
>> 1 file changed, 16 insertions(+)
>>
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index 20b4b71a1a07..18107db118d4 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers);
>> */
>> int __read_mostly suppress_printk;
>>
>> +/*
>> + * During panic, heavy printk by other CPUs can delay the
>> + * panic and risk deadlock on console resources.
>> + */
>> +int __read_mostly suppress_panic_printk;
>> +
>> #ifdef CONFIG_LOCKDEP
>> static struct lockdep_map console_lock_dep_map = {
>> .name = "console_lock"
>> @@ -2228,6 +2234,10 @@ asmlinkage int vprintk_emit(int facility, int level,
>> if (unlikely(suppress_printk))
>> return 0;
>>
>> + if (unlikely(suppress_panic_printk) &&
>> + atomic_read(&panic_cpu) != raw_smp_processor_id())
>> + return 0;
>> +
>> if (level == LOGLEVEL_SCHED) {
>> level = LOGLEVEL_DEFAULT;
>> in_sched = true;
>> @@ -2613,6 +2623,7 @@ void console_unlock(void)
>> {
>> static char ext_text[CONSOLE_EXT_LOG_MAX];
>> static char text[CONSOLE_LOG_MAX];
>> + static int panic_console_dropped;
>> unsigned long flags;
>> bool do_cond_resched, retry;
>> struct printk_info info;
>> @@ -2667,6 +2678,11 @@ void console_unlock(void)
>> if (console_seq != r.info->seq) {
>> console_dropped += r.info->seq - console_seq;
>> console_seq = r.info->seq;
>> + if (panic_in_progress() && panic_console_dropped++ > 10) {
>> + suppress_panic_printk = 1;
>> + pr_warn("Too many dropped messages. "
>> + "Supress messages on non-panic CPUs to prevent livelock.\n");
>
> It looks like the message might be printed more times when
> panic_console_dropped++ > 10.
>
> In theory, no message can be lost after we disable printk on another
> CPUs. But the code might evolve in the future. Let's make it
> more error-proof.
>
> We could use (panic_console_dropped++ == 10) or pr_warn_once() or
> both.
>
> I prefer using pr_warn_once() because it looks the most error-proof.
>
>
> Nit: printk() has exceptions from the 80 chars/line rule.
> The message string should be on a single line. It helps
> to find it using "git grep". I think that even checkpatch.pl
> handles this correctly.
>
>
> With pr_warn_once() and message in single line:
>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
>
> Best Regards,
> Petr
>
>
> PS: I could fix the two problems when pushing to git. But there
> is still time to send v3. I have vacation the following week
> with limited internet access. I am not going to rush it into
> linux before I leave, ...
>

Thank you Petr, I will go ahead and resolve things in this patch, and
the others in a v3 soon. Regardless, no need to rush, please enjoy your
vacation!

Thanks for all the review and guidance!
Stephen

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