[PATCH v4 2/3] printk: set may_schedule for some of console_trylock callers

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

console_unlock() allows to cond_resched() if its caller has
set `console_may_schedule' to 1, since
'commit 8d91f8b15361 ("printk: do cond_resched() between lines while
outputting to consoles")'.

The rules are:
-- console_lock() always sets `console_may_schedule' to 1
-- console_trylock() always sets `console_may_schedule' to 0

However, console_trylock() callers (among them is printk()) do
not always call printk() from atomic contexts, and some of them
can cond_resched() in console_unlock(), so console_trylock()
can set `console_may_schedule' to 1 for such processes.

For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock()
always sets `console_may_schedule' to 0.

It's possible to drop explicit preempt_disable()/preempt_enable()
in vprintk_emit(), because console_unlock() and console_trylock()
are now smart enough:
a) console_unlock() does not cond_resched() when it's unsafe
  (console_trylock() takes care of that)
b) console_unlock() does can_use_console() check.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0d65a81..9f0c3e1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1769,20 +1769,12 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (!in_sched) {
 		lockdep_off();
 		/*
-		 * Disable preemption to avoid being preempted while holding
-		 * console_sem which would prevent anyone from printing to
-		 * console
-		 */
-		preempt_disable();
-
-		/*
 		 * 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())
 			console_unlock();
-		preempt_enable();
 		lockdep_on();
 	}
 
@@ -2115,7 +2107,20 @@ int console_trylock(void)
 		return 0;
 	}
 	console_locked = 1;
-	console_may_schedule = 0;
+	/*
+	 * When PREEMPT_COUNT disabled we can't reliably detect if it's
+	 * safe to schedule (e.g. calling printk while holding a spin_lock),
+	 * because preempt_disable()/preempt_enable() are just barriers there
+	 * and preempt_count() is always 0.
+	 *
+	 * RCU read sections have a separate preemption counter when
+	 * PREEMPT_RCU enabled thus we must take extra care and check
+	 * rcu_preempt_depth(), otherwise RCU read sections modify
+	 * preempt_count().
+	 */
+	console_may_schedule = !oops_in_progress &&
+			preemptible() &&
+			!rcu_preempt_depth();
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
-- 
2.7.1