Subject: printk: use seqcount_latch for console_seq From: John Ogness Date: Mon Nov 30 01:42:05 2020 +0106 From: John Ogness In preparation for atomic printing, change @console_seq to use seqcount_latch so that it can be read without requiring @console_sem. Signed-off-by: John Ogness Signed-off-by: Thomas Gleixner --- kernel/printk/printk.c | 73 +++++++++++++++++++++++++++---------------------- 1 file changed, 41 insertions(+), 32 deletions(-) --- --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -362,9 +362,7 @@ 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; @@ -374,6 +372,17 @@ struct latched_seq { }; /* + * 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 * access a valid value. Writers are synchronized by @syslog_lock. @@ -436,7 +445,7 @@ 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); @@ -2278,9 +2287,9 @@ 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; @@ -2608,7 +2617,7 @@ 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(); @@ -2652,12 +2661,14 @@ 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)) { @@ -2666,13 +2677,13 @@ 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; } @@ -2693,7 +2704,7 @@ 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() @@ -2721,9 +2732,6 @@ 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(); @@ -2733,7 +2741,7 @@ 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; } @@ -2785,18 +2793,19 @@ 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)); + } else { + /* + * Another context is holding the console lock and + * @console_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 @console_may_schedule is cleared. + */ + console_may_schedule = 0; + } console_unlock(); } @@ -3032,11 +3041,11 @@ void register_console(struct console *ne * ignores console_lock. */ exclusive_console = newcon; - exclusive_console_stop_seq = console_seq; + exclusive_console_stop_seq = latched_seq_read_nolock(&console_seq); /* Get a consistent copy of @syslog_seq. */ mutex_lock(&syslog_lock); - console_seq = syslog_seq; + latched_seq_write(&console_seq, syslog_seq); mutex_unlock(&syslog_lock); } console_unlock();