Revert "printk: make reading the kernel log flush pending lines"

This reverts commit bfd8d3f23b51018388be0411ccbc2d56277fe294.

It turns out that this flushes things much too aggressiverly, and causes
lines to break up when the system logger races with new continuation
lines being printed.

There's a pending patch to make printk() flushing much more
straightforward, but it's too invasive for 4.9, so in the meantime let's
just not make the system message logging flush continuation lines.
They'll be flushed by the final newline anyway.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/printk.c

@@ -783,8 +783,6 @@ static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
 	return ret;
 }
 
-static void cont_flush(void);
-
 static ssize_t devkmsg_read(struct file *file, char __user *buf,
 			    size_t count, loff_t *ppos)
 {
@@ -800,7 +798,6 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 	if (ret)
 		return ret;
 	raw_spin_lock_irq(&logbuf_lock);
-	cont_flush();
 	while (user->seq == log_next_seq) {
 		if (file->f_flags & O_NONBLOCK) {
 			ret = -EAGAIN;
@@ -863,7 +860,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
 		return -ESPIPE;
 
 	raw_spin_lock_irq(&logbuf_lock);
-	cont_flush();
 	switch (whence) {
 	case SEEK_SET:
 		/* the first record */
@@ -902,7 +898,6 @@ static unsigned int devkmsg_poll(struct file *file, poll_table *wait)
 	poll_wait(file, &log_wait, wait);
 
 	raw_spin_lock_irq(&logbuf_lock);
-	cont_flush();
 	if (user->seq < log_next_seq) {
 		/* return error when data has vanished underneath us */
 		if (user->seq < log_first_seq)
@@ -1289,7 +1284,6 @@ static int syslog_print(char __user *buf, int size)
 		size_t skip;
 
 		raw_spin_lock_irq(&logbuf_lock);
-		cont_flush();
 		if (syslog_seq < log_first_seq) {
 			/* messages are gone, move to first one */
 			syslog_seq = log_first_seq;
@@ -1349,7 +1343,6 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		return -ENOMEM;
 
 	raw_spin_lock_irq(&logbuf_lock);
-	cont_flush();
 	if (buf) {
 		u64 next_seq;
 		u64 seq;
@@ -1511,7 +1504,6 @@ int do_syslog(int type, char __user *buf, int len, int source)
 	/* Number of chars in the log buffer */
 	case SYSLOG_ACTION_SIZE_UNREAD:
 		raw_spin_lock_irq(&logbuf_lock);
-		cont_flush();
 		if (syslog_seq < log_first_seq) {
 			/* messages are gone, move to first one */
 			syslog_seq = log_first_seq;
@@ -3028,7 +3020,6 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 		dumper->active = true;
 
 		raw_spin_lock_irqsave(&logbuf_lock, flags);
-		cont_flush();
 		dumper->cur_seq = clear_seq;
 		dumper->cur_idx = clear_idx;
 		dumper->next_seq = log_next_seq;
@@ -3119,7 +3110,6 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 	bool ret;
 
 	raw_spin_lock_irqsave(&logbuf_lock, flags);
-	cont_flush();
 	ret = kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
 	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 
@@ -3162,7 +3152,6 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 		goto out;
 
 	raw_spin_lock_irqsave(&logbuf_lock, flags);
-	cont_flush();
 	if (dumper->cur_seq < log_first_seq) {
 		/* messages are gone, move to first available one */
 		dumper->cur_seq = log_first_seq;