printk: lock/unlock console only for new logbuf entries

Prior to commit 5c2992ee7fd8a29 ("printk: remove console flushing special
cases for partial buffered lines") we would do console_cont_flush()
for each pr_cont() to print cont fragments, so console_unlock() would
actually print data:

	pr_cont();
	 console_lock();
	 console_unlock()
	  console_cont_flush(); // print cont fragment
	...
	pr_cont();
	 console_lock();
	 console_unlock()
	  console_cont_flush(); // print cont fragment

We don't do console_cont_flush() anymore, so when we do pr_cont()
console_unlock() does nothing (unless we flushed the cont buffer):

	pr_cont();
	 console_lock();
	 console_unlock();      // noop
	...
	pr_cont();
	 console_lock();
	 console_unlock();      // noop
	...
	pr_cont();
	  cont_flush();
	    console_lock();
	    console_unlock();   // print data

We also wakeup klogd purposelessly for pr_cont() output - un-flushed
cont buffer is not stored in log_buf; there is nothing to pull.

Thus we can console_lock()/console_unlock()/wake_up_klogd() only when
we know that we log_store()-ed a message and there is something to
print to the consoles/syslog.

Link: http://lkml.kernel.org/r/20181002023836.4487-3-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f717656c0fac..e9a7e50ed60a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1890,8 +1890,9 @@ asmlinkage int vprintk_emit(int facility, int level,
                             const char *fmt, va_list args)
 {
         int printed_len;
-        bool in_sched = false;
+        bool in_sched = false, pending_output;
         unsigned long flags;
+        u64 curr_log_seq;
 
         if (level == LOGLEVEL_SCHED) {
                 level = LOGLEVEL_DEFAULT;
@@ -1903,11 +1904,13 @@ asmlinkage int vprintk_emit(int facility, int level,
 
         /* This stops the holder of console_sem just where we want him */
         logbuf_lock_irqsave(flags);
+        curr_log_seq = log_next_seq;
         printed_len = vprintk_store(facility, level, dict, dictlen, fmt, args);
+        pending_output = (curr_log_seq != log_next_seq);
         logbuf_unlock_irqrestore(flags);
 
         /* If called from the scheduler, we can not call up(). */
-        if (!in_sched) {
+        if (!in_sched && pending_output) {
                 /*
                  * Disable preemption to avoid being preempted while holding
                  * console_sem which would prevent anyone from printing to
@@ -1924,7 +1927,8 @@ asmlinkage int vprintk_emit(int facility, int level,
                 preempt_enable();
         }
 
-        wake_up_klogd();
+        if (pending_output)
+                wake_up_klogd();
         return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);