Re: [PATCH printk v1 06/10] printk: use seqcount_latch for console_seq

Petr Mladek
2021-08-05T12:17:06Z

On Tue 2021-08-03 15:18:57, John Ogness wrote:
> In preparation for synchronous printing, change @console_seq to use
> seqcount_latch so that it can be read without requiring @console_sem.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
> kernel/printk/printk.c | 73 ++++++++++++++++++++++++------------------
> 1 file changed, 41 insertions(+), 32 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d07d98c1e846..f8f46d9fba9b 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -489,9 +489,7 @@ static u64 syslog_seq;
> static size_t syslog_partial;
> static bool syslog_time;
>
> -/* All 3 protected by @console_sem. */
> -/* the next printk record to write to the console */
> -static u64 console_seq;
> +/* Both protected by @console_sem. */
> static u64 exclusive_console_stop_seq;
> static unsigned long console_dropped;
>
> @@ -500,6 +498,17 @@ struct latched_seq {
> u64 val[2];
> };
>
> +/*
> + * The next printk record to write to the console. There are two
> + * copies (updated with seqcount_latch) so that reads can locklessly
> + * access a valid value. Writers are synchronized by @console_sem.
> + */
> +static struct latched_seq console_seq = {
> + .latch = SEQCNT_LATCH_ZERO(console_seq.latch),
> + .val[0] = 0,
> + .val[1] = 0,
> +};
> +
> /*
> * The next printk record to read after the last 'clear' command. There are
> * two copies (updated with seqcount_latch) so that reads can locklessly
> @@ -563,7 +572,7 @@ bool printk_percpu_data_ready(void)
> return __printk_percpu_data_ready;
> }
>
> -/* Must be called under syslog_lock. */
> +/* Must be called under associated write-protection lock. */
> static void latched_seq_write(struct latched_seq *ls, u64 val)
> {
> raw_write_seqcount_latch(&ls->latch);
> @@ -2405,9 +2414,9 @@ EXPORT_SYMBOL(_printk);
>
> #define prb_read_valid(rb, seq, r) false
> #define prb_first_valid_seq(rb) 0
> +#define latched_seq_read_nolock(seq) 0
> +#define latched_seq_write(dst, src)
>
> -static u64 syslog_seq;
> -static u64 console_seq;
> static u64 exclusive_console_stop_seq;
> static unsigned long console_dropped;
>
> @@ -2735,7 +2744,7 @@ void console_unlock(void)
> bool do_cond_resched, retry;
> struct printk_info info;
> struct printk_record r;
> - u64 __maybe_unused next_seq;
> + u64 seq;
>
> if (console_suspended) {
> up_console_sem();
> @@ -2779,12 +2788,14 @@ void console_unlock(void)
> size_t len;
>
> skip:
> - if (!prb_read_valid(prb, console_seq, &r))
> + seq = latched_seq_read_nolock(&console_seq);
> + if (!prb_read_valid(prb, seq, &r))
> break;
>
> - if (console_seq != r.info->seq) {
> - console_dropped += r.info->seq - console_seq;
> - console_seq = r.info->seq;
> + if (seq != r.info->seq) {
> + console_dropped += r.info->seq - seq;
> + latched_seq_write(&console_seq, r.info->seq);
> + seq = r.info->seq;
> }
>
> if (suppress_message_printing(r.info->level)) {
> @@ -2793,13 +2804,13 @@ void console_unlock(void)
> * directly to the console when we received it, and
> * record that has level above the console loglevel.
> */
> - console_seq++;
> + latched_seq_write(&console_seq, seq + 1);
> goto skip;
> }
>
> /* Output to all consoles once old messages replayed. */
> if (unlikely(exclusive_console &&
> - console_seq >= exclusive_console_stop_seq)) {
> + seq >= exclusive_console_stop_seq)) {
> exclusive_console = NULL;
> }
>
> @@ -2820,7 +2831,7 @@ void console_unlock(void)
> len = record_print_text(&r,
> console_msg_format & MSG_FORMAT_SYSLOG,
> printk_time);
> - console_seq++;
> + latched_seq_write(&console_seq, seq + 1);
>
> /*
> * While actively printing out messages, if another printk()
> @@ -2848,9 +2859,6 @@ void console_unlock(void)
> cond_resched();
> }
>
> - /* Get consistent value of the next-to-be-used sequence number. */
> - next_seq = console_seq;
> -
> console_locked = 0;
> up_console_sem();
>
> @@ -2860,7 +2868,7 @@ void console_unlock(void)
> * there's a new owner and the console_unlock() from them will do the
> * flush, no worries.
> */
> - retry = prb_read_valid(prb, next_seq, NULL);
> + retry = prb_read_valid(prb, latched_seq_read_nolock(&console_seq), NULL);
> if (retry && console_trylock())
> goto again;
> }
> @@ -2912,18 +2920,19 @@ void console_unblank(void)
> */
> void console_flush_on_panic(enum con_flush_mode mode)
> {
> - /*
> - * 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;
> -
> - if (mode == CONSOLE_REPLAY_ALL)
> - console_seq = prb_first_valid_seq(prb);
> + if (console_trylock()) {
> + if (mode == CONSOLE_REPLAY_ALL)
> + latched_seq_write(&console_seq, prb_first_valid_seq(prb));

I am scratching my head about this. Of course, latched_seq_write() does
not guarantee the result when the console lock it taken by another process.
But console_lock(), called below, will call latched_seq_write()
anyway.

Also CONSOLE_REPLAY_ALL is used by panic_print_sys_info().
It is called the following way:

void panic(const char *fmt, ...)
{
[...]
debug_locks_off();
console_flush_on_panic(CONSOLE_FLUSH_PENDING);

panic_print_sys_info();
[...]
}

On one hand, console_flush_on_panic(CONSOLE_FLUSH_PENDING) will
most likely take over the console lock even when it was taken
by another CPU before. And the 2nd console_flush_on_panic()
called from panic_print_sys_info() will not even notice.

On the other hand, CONSOLE_REPLAY_ALL would not even try to
reply the log when the console log was not available.

The risk of broken console_seq is neglible. console_unlock()
should be safe even with invalid console_seq.

My opinion:

I suggest to keep the original logic and maybe add some comment:

void console_flush_on_panic(enum con_flush_mode mode)
{
/*
* 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;

/*
* latched_seq_write() does not guarantee consistent values
* when console_trylock() failed. But this is the best effort.
* console_unlock() will update anyway console_seq. prb_read_valid()
* handles even invalid sequence numbers.
*/
if (mode == CONSOLE_REPLAY_ALL)
latched_seq_write(&console_seq, prb_first_valid_seq(prb));

console_unlock();
}

Best Regards,
Petr