[PATCH] printk: stop spining waiter when console resume to flush prb

[Luo Jiaxing]

Some threads still call printk() for printing when resume_console() is
being executed. In practice, the printk() is executed for a period of time
and then returned. The duration is determined by the number of prints
cached in the prb during the suspend/resume process. At the same time,
resume_console() returns quickly.

Base on owner/waiter machanism, the frist one who fail to lock console will
become waiter, and start spining. When current owner finish print one
informance, if a waiter is waitting, owner will give up and let waiter
become a new owner. New owner need to flush the whole prb unitl prb empty
or another new waiter come and take the job from him.

So the first waiter after resume_console() will take seconds to help to
flush prb, but driver which call printk() may be bothered by this. New
a flag to mark resume flushing prb. When the console resume, before the
prb is empty, stop to set a new waiter temporarily.

Signed-off-by: Luo Jiaxing <luojiaxing@xxxxxxxxxx>
---
 kernel/printk/printk.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 575a34b..2c680a5 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -287,6 +287,9 @@ EXPORT_SYMBOL(console_set_on_cmdline);
 /* Flag: console code may call schedule() */
 static int console_may_schedule;
 
+/* Flags: console flushing prb when resume */
+static atomic_t console_resume_flush_prb = ATOMIC_INIT(0);
+
 enum con_msg_format_flags {
 	MSG_FORMAT_DEFAULT	= 0,
 	MSG_FORMAT_SYSLOG	= (1 << 0),
@@ -1781,7 +1784,8 @@ static int console_trylock_spinning(void)
 	raw_spin_lock(&console_owner_lock);
 	owner = READ_ONCE(console_owner);
 	waiter = READ_ONCE(console_waiter);
-	if (!waiter && owner && owner != current) {
+	if (!waiter && owner && owner != current &&
+	    !atomic_read(&console_resume_flush_prb)) {
 		WRITE_ONCE(console_waiter, true);
 		spin = true;
 	}
@@ -2355,6 +2359,7 @@ void resume_console(void)
 	if (!console_suspend_enabled)
 		return;
 	down_console_sem();
+	atomic_set(&console_resume_flush_prb, 1);
 	console_suspended = 0;
 	console_unlock();
 }
@@ -2592,6 +2597,8 @@ void console_unlock(void)
 	raw_spin_unlock(&logbuf_lock);
 
 	up_console_sem();
+	if (atomic_read(&console_resume_flush_prb))
+		atomic_set(&console_resume_flush_prb, 0);
 
 	/*
 	 * Someone could have filled up the buffer again, so re-check if there's
-- 
2.7.4