From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 19 Sep 2012 14:50:37 +0200
Subject: printk: Make rt aware
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 8414727d39438575d33e6c150ade0a1e3f3a78ca
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

Drop the lock before calling the console driver and do not disable
interrupts while printing to a serial console.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/printk/printk.c |   30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1648,6 +1648,7 @@ static DEFINE_RAW_SPINLOCK(console_owner
 static struct task_struct *console_owner;
 static bool console_waiter;
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /**
  * printk_bust_locks - forcibly reset all printk-related locks
  *
@@ -1724,6 +1725,7 @@ static int console_lock_spinning_disable
 	mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
 	return 1;
 }
+#endif
 
 /**
  * console_trylock_spinning - try to get console_lock by busy waiting
@@ -1755,6 +1757,9 @@ static int console_trylock_spinning(void
 	if (panic_in_progress())
 		return 0;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+		return 0;
+
 	printk_safe_enter_irqsave(flags);
 
 	raw_spin_lock(&console_owner_lock);
@@ -1814,6 +1819,7 @@ static void call_console_drivers(const c
 	if (!console_drivers)
 		return;
 
+	migrate_disable();
 	for_each_console(con) {
 		if (exclusive_console && con != exclusive_console)
 			continue;
@@ -1829,6 +1835,7 @@ static void call_console_drivers(const c
 		else
 			con->write(con, text, len);
 	}
+	migrate_enable();
 }
 
 int printk_delay_msec __read_mostly;
@@ -2011,20 +2018,30 @@ asmlinkage int vprintk_emit(int facility
 
 	/* If called from the scheduler, we can not call up(). */
 	if (!in_sched) {
+			int may_trylock = 1;
+
+#ifdef CONFIG_PREEMPT_RT_FULL
+		/*
+		 * we can't take a sleeping lock with IRQs or preeption
+		 * disabled so we can't print in these contexts
+		 */
+		if (!(preempt_count() == 0 && !irqs_disabled()))
+			may_trylock = 0;
+#endif
 		/*
 		 * Disable preemption to avoid being preempted while holding
 		 * console_sem which would prevent anyone from printing to
 		 * console
 		 */
-		preempt_disable();
+		preempt_disable_nort();
 		/*
 		 * Try to acquire and then immediately release the console
 		 * semaphore.  The release will print out buffers and wake up
 		 * /dev/kmsg and syslog() users.
 		 */
-		if (console_trylock_spinning())
+		if (may_trylock && console_trylock_spinning())
 			console_unlock();
-		preempt_enable();
+		preempt_enable_nort();
 	}
 
 	return printed_len;
@@ -2480,6 +2497,10 @@ skip:
 		console_seq++;
 		raw_spin_unlock(&logbuf_lock);
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+		printk_safe_exit_irqrestore(flags);
+		call_console_drivers(ext_text, ext_len, text, len);
+#else
 		/*
 		 * While actively printing out messages, if another printk()
 		 * were to occur on another CPU, it may wait for this one to
@@ -2498,6 +2519,7 @@ skip:
 		}
 
 		printk_safe_exit_irqrestore(flags);
+#endif
 
 		if (do_cond_resched)
 			cond_resched();
@@ -2527,7 +2549,9 @@ skip:
 	if (retry && console_trylock())
 		goto again;
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 out:
+#endif
 	if (wake_klogd)
 		wake_up_klogd();
 }