[RFC][PATCHv2 7/7] printk: remove zap_locks() function

From: Sergey Senozhatsky
Date: Fri Sep 30 2016 - 11:19:43 EST


We use alt_printk now which makes printk-recursion detection code
in vprintk_emit() is unreachable. The tricky thing here is that,
apart from detecting and reporting printk recursions, that code also
used to zap_lockc() in case of panic. However, zap_locks() does not
look to be needed anymore:

1) Since 08d78658f393 ('panic: release stale console lock to always get
the logbuf printed out') panic flushing of `logbuf' to console ignores
the state of `console_sem' by doing
panic()
console_trylock();
console_unlock();

2) Since cf9b1106c81c ('printk/nmi: flush NMI messages on the system
panic') panic attempts to zap the `logbuf_lock' spin_lock to
successfully flush nmi messages to `logbuf'.

Basically, it seems that we either already do what zap_locks() used to
do but in other places or we ignore the state of the lock. May be we
still would want to do sema_init() in alt_printk_flush_on_panic(), just
in case.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
kernel/printk/printk.c | 63 --------------------------------------------------
1 file changed, 63 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b99d6b3..fb2d26b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1600,28 +1600,6 @@ static void call_console_drivers(int level,
}
}

-/*
- * Zap console related locks when oopsing.
- * To leave time for slow consoles to print a full oops,
- * only zap at most once every 30 seconds.
- */
-static void zap_locks(void)
-{
- static unsigned long oops_timestamp;
-
- if (time_after_eq(jiffies, oops_timestamp) &&
- !time_after(jiffies, oops_timestamp + 30 * HZ))
- return;
-
- oops_timestamp = jiffies;
-
- debug_locks_off();
- /* If a crash is occurring, make sure we can't deadlock */
- raw_spin_lock_init(&logbuf_lock);
- /* And make sure that we print immediately */
- sema_init(&console_sem, 1);
-}
-
int printk_delay_msec __read_mostly;

static inline void printk_delay(void)
@@ -1748,18 +1726,13 @@ asmlinkage int vprintk_emit(int facility, int level,
const char *dict, size_t dictlen,
const char *fmt, va_list args)
{
- static bool recursion_bug;
static char textbuf[LOG_LINE_MAX];
char *text = textbuf;
size_t text_len = 0;
enum log_flags lflags = 0;
- unsigned long flags;
- int this_cpu;
int printed_len = 0;
int nmi_message_lost;
bool in_sched = false;
- /* cpu currently holding logbuf_lock in this function */
- static unsigned int logbuf_cpu = UINT_MAX;

if (level == LOGLEVEL_SCHED) {
level = LOGLEVEL_DEFAULT;
@@ -1769,43 +1742,9 @@ asmlinkage int vprintk_emit(int facility, int level,
boot_delay_msec(level);
printk_delay();

- local_irq_save(flags);
- this_cpu = smp_processor_id();
-
- /*
- * Ouch, printk recursed into itself!
- */
- if (unlikely(logbuf_cpu == this_cpu)) {
- /*
- * If a crash is occurring during printk() on this CPU,
- * then try to get the crash message out but make sure
- * we can't deadlock. Otherwise just return to avoid the
- * recursion and return - but flag the recursion so that
- * it can be printed at the next appropriate moment:
- */
- if (!oops_in_progress && !lockdep_recursing(current)) {
- recursion_bug = true;
- local_irq_restore(flags);
- return 0;
- }
- zap_locks();
- }
-
alt_printk_enter();
/* This stops the holder of console_sem just where we want him */
raw_spin_lock(&logbuf_lock);
- logbuf_cpu = this_cpu;
-
- if (unlikely(recursion_bug)) {
- static const char recursion_msg[] =
- "BUG: recent printk recursion!";
-
- recursion_bug = false;
- /* emit KERN_CRIT message */
- printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
- NULL, 0, recursion_msg,
- strlen(recursion_msg));
- }

nmi_message_lost = get_nmi_message_lost();
if (unlikely(nmi_message_lost)) {
@@ -1898,10 +1837,8 @@ asmlinkage int vprintk_emit(int facility, int level,
dict, dictlen, text, text_len);
}

- logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock);
alt_printk_exit();
- local_irq_restore(flags);

/* If called from the scheduler, we can not call up(). */
if (!in_sched) {
--
2.10.0.372.g6fe1b14