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Subject[RFC PATCH v1 12/25] printk: minimize console locking implementation
Date
Since printing of the printk buffer is now handled by the printk
kthread, minimize the console locking functions to just handle
locking of the console.

NOTE: With this console_flush_on_panic will no longer flush.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 255 +------------------------------------------------
1 file changed, 1 insertion(+), 254 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 073ff9fd6872..ece54c24ea0d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -209,19 +209,7 @@ static int nr_ext_console_drivers;

static int __down_trylock_console_sem(unsigned long ip)
{
- int lock_failed;
- unsigned long flags;
-
- /*
- * Here and in __up_console_sem() we need to be in safe mode,
- * because spindump/WARN/etc from under console ->lock will
- * deadlock in printk()->down_trylock_console_sem() otherwise.
- */
- printk_safe_enter_irqsave(flags);
- lock_failed = down_trylock(&console_sem);
- printk_safe_exit_irqrestore(flags);
-
- if (lock_failed)
+ if (down_trylock(&console_sem))
return 1;
mutex_acquire(&console_lock_dep_map, 0, 1, ip);
return 0;
@@ -230,13 +218,9 @@ static int __down_trylock_console_sem(unsigned long ip)

static void __up_console_sem(unsigned long ip)
{
- unsigned long flags;
-
mutex_release(&console_lock_dep_map, 1, ip);

- printk_safe_enter_irqsave(flags);
up(&console_sem);
- printk_safe_exit_irqrestore(flags);
}
#define up_console_sem() __up_console_sem(_RET_IP_)

@@ -1498,82 +1482,6 @@ static void format_text(struct printk_log *msg, u64 seq,
}

/*
- * Special console_lock variants that help to reduce the risk of soft-lockups.
- * They allow to pass console_lock to another printk() call using a busy wait.
- */
-
-#ifdef CONFIG_LOCKDEP
-static struct lockdep_map console_owner_dep_map = {
- .name = "console_owner"
-};
-#endif
-
-static DEFINE_RAW_SPINLOCK(console_owner_lock);
-static struct task_struct *console_owner;
-static bool console_waiter;
-
-/**
- * console_lock_spinning_enable - mark beginning of code where another
- * thread might safely busy wait
- *
- * This basically converts console_lock into a spinlock. This marks
- * the section where the console_lock owner can not sleep, because
- * there may be a waiter spinning (like a spinlock). Also it must be
- * ready to hand over the lock at the end of the section.
- */
-static void console_lock_spinning_enable(void)
-{
- raw_spin_lock(&console_owner_lock);
- console_owner = current;
- raw_spin_unlock(&console_owner_lock);
-
- /* The waiter may spin on us after setting console_owner */
- spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
-}
-
-/**
- * console_lock_spinning_disable_and_check - mark end of code where another
- * thread was able to busy wait and check if there is a waiter
- *
- * This is called at the end of the section where spinning is allowed.
- * It has two functions. First, it is a signal that it is no longer
- * safe to start busy waiting for the lock. Second, it checks if
- * there is a busy waiter and passes the lock rights to her.
- *
- * Important: Callers lose the lock if there was a busy waiter.
- * They must not touch items synchronized by console_lock
- * in this case.
- *
- * Return: 1 if the lock rights were passed, 0 otherwise.
- */
-static int console_lock_spinning_disable_and_check(void)
-{
- int waiter;
-
- raw_spin_lock(&console_owner_lock);
- waiter = READ_ONCE(console_waiter);
- console_owner = NULL;
- raw_spin_unlock(&console_owner_lock);
-
- if (!waiter) {
- spin_release(&console_owner_dep_map, 1, _THIS_IP_);
- return 0;
- }
-
- /* The waiter is now free to continue */
- WRITE_ONCE(console_waiter, false);
-
- spin_release(&console_owner_dep_map, 1, _THIS_IP_);
-
- /*
- * Hand off console_lock to waiter. The waiter will perform
- * the up(). After this, the waiter is the console_lock owner.
- */
- mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
- return 1;
-}
-
-/*
* Call the console drivers, asking them to write out
* log_buf[start] to log_buf[end - 1].
* The console_lock must be held.
@@ -1830,8 +1738,6 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
static ssize_t msg_print_ext_body(char *buf, size_t size,
char *dict, size_t dict_len,
char *text, size_t text_len) { return 0; }
-static void console_lock_spinning_enable(void) { }
-static int console_lock_spinning_disable_and_check(void) { return 0; }
static void call_console_drivers(const char *ext_text, size_t ext_len,
const char *text, size_t len) {}
static size_t msg_print_text(const struct printk_log *msg, bool syslog,
@@ -2066,35 +1972,6 @@ int is_console_locked(void)
{
return console_locked;
}
-EXPORT_SYMBOL(is_console_locked);
-
-/*
- * Check if we have any console that is capable of printing while cpu is
- * booting or shutting down. Requires console_sem.
- */
-static int have_callable_console(void)
-{
- struct console *con;
-
- for_each_console(con)
- if ((con->flags & CON_ENABLED) &&
- (con->flags & CON_ANYTIME))
- return 1;
-
- return 0;
-}
-
-/*
- * Can we actually use the console at this time on this cpu?
- *
- * Console drivers may assume that per-cpu resources have been allocated. So
- * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
- * call them until this CPU is officially up.
- */
-static inline int can_use_console(void)
-{
- return cpu_online(raw_smp_processor_id()) || have_callable_console();
-}

/**
* console_unlock - unlock the console system
@@ -2102,147 +1979,17 @@ static inline int can_use_console(void)
* Releases the console_lock which the caller holds on the console system
* and the console driver list.
*
- * While the console_lock was held, console output may have been buffered
- * by printk(). If this is the case, console_unlock(); emits
- * the output prior to releasing the lock.
- *
- * If there is output waiting, we wake /dev/kmsg and syslog() users.
- *
* console_unlock(); may be called from any context.
*/
void console_unlock(void)
{
- static char ext_text[CONSOLE_EXT_LOG_MAX];
- static char text[LOG_LINE_MAX + PREFIX_MAX];
- unsigned long flags;
- bool do_cond_resched, retry;
-
if (console_suspended) {
up_console_sem();
return;
}

- /*
- * Console drivers are called with interrupts disabled, so
- * @console_may_schedule should be cleared before; however, we may
- * end up dumping a lot of lines, for example, if called from
- * console registration path, and should invoke cond_resched()
- * between lines if allowable. Not doing so can cause a very long
- * scheduling stall on a slow console leading to RCU stall and
- * softlockup warnings which exacerbate the issue with more
- * messages practically incapacitating the system.
- *
- * console_trylock() is not able to detect the preemptive
- * context reliably. Therefore the value must be stored before
- * and cleared after the the "again" goto label.
- */
- do_cond_resched = console_may_schedule;
-again:
- console_may_schedule = 0;
-
- /*
- * We released the console_sem lock, so we need to recheck if
- * cpu is online and (if not) is there at least one CON_ANYTIME
- * console.
- */
- if (!can_use_console()) {
- console_locked = 0;
- up_console_sem();
- return;
- }
-
- for (;;) {
- struct printk_log *msg;
- size_t ext_len = 0;
- size_t len;
-
- 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 */
- console_seq = log_first_seq;
- console_idx = log_first_idx;
- } else {
- len = 0;
- }
-skip:
- if (console_seq == log_next_seq)
- break;
-
- msg = log_from_idx(console_idx);
- if (suppress_message_printing(msg->level)) {
- /*
- * Skip record we have buffered and already printed
- * directly to the console when we received it, and
- * record that has level above the console loglevel.
- */
- console_idx = log_next(console_idx);
- console_seq++;
- goto skip;
- }
-
- len += msg_print_text(msg,
- console_msg_format & MSG_FORMAT_SYSLOG,
- printk_time, text + len, sizeof(text) - len);
- if (nr_ext_console_drivers) {
- ext_len = msg_print_ext_header(ext_text,
- sizeof(ext_text),
- msg, console_seq);
- ext_len += msg_print_ext_body(ext_text + ext_len,
- sizeof(ext_text) - ext_len,
- log_dict(msg), msg->dict_len,
- log_text(msg), msg->text_len);
- }
- console_idx = log_next(console_idx);
- console_seq++;
- raw_spin_unlock(&logbuf_lock);
-
- /*
- * While actively printing out messages, if another printk()
- * were to occur on another CPU, it may wait for this one to
- * finish. This task can not be preempted if there is a
- * waiter waiting to take over.
- */
- console_lock_spinning_enable();
-
- stop_critical_timings(); /* don't trace print latency */
- //call_console_drivers(ext_text, ext_len, text, len);
- start_critical_timings();
-
- if (console_lock_spinning_disable_and_check()) {
- printk_safe_exit_irqrestore(flags);
- return;
- }
-
- printk_safe_exit_irqrestore(flags);
-
- if (do_cond_resched)
- cond_resched();
- }
-
console_locked = 0;
-
- raw_spin_unlock(&logbuf_lock);
-
up_console_sem();
-
- /*
- * Someone could have filled up the buffer again, so re-check if there's
- * something to flush. In case we cannot trylock the console_sem again,
- * there's a new owner and the console_unlock() from them will do the
- * flush, no worries.
- */
- raw_spin_lock(&logbuf_lock);
- retry = console_seq != log_next_seq;
- raw_spin_unlock(&logbuf_lock);
- printk_safe_exit_irqrestore(flags);
-
- if (retry && console_trylock())
- goto again;
}
EXPORT_SYMBOL(console_unlock);

--
2.11.0
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