Messages in this thread | | | Date | Thu, 5 Aug 2021 14:16:59 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v1 06/10] printk: use seqcount_latch for console_seq |
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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
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