[RFC][PATCHv6 09/12] printk: do not cond_resched() when we can offload

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

console_unlock() may sleep with console_sem locked, which is a bit
counter intuitive: we neither print pending logbuf messages to the
serial console, nor let anyone else to do it for us.

With printing offloading enabled, however, we can disable preemption,
because we know for sure how long we can stay in console_unlock() and
that eventually we will offload to another task.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 4d91182e25e3..2a12d4c02da1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2074,6 +2074,7 @@ int printk_emergency_end_sync(void) { return 0; }
 EXPORT_SYMBOL_GPL(printk_emergency_end_sync);
 
 static bool should_handoff_printing(u64 printing_start_ts) { return false; }
+static bool printk_offloading_enabled(void) { return false; }
 #endif /* CONFIG_PRINTK */
 
 #ifdef CONFIG_EARLY_PRINTK
@@ -2385,6 +2386,14 @@ void console_unlock(void)
 	 * and cleared after the the "again" goto label.
 	 */
 	do_cond_resched = console_may_schedule;
+	/*
+	 * Forbid scheduling under the console_sem lock when offloading
+	 * is enabled. Scheduling will just slow down the print out in
+	 * this case.
+	 */
+	if (printk_offloading_enabled())
+		do_cond_resched = 0;
+
 again:
 	console_may_schedule = 0;
 
@@ -2400,6 +2409,7 @@ void console_unlock(void)
 		return;
 	}
 
+	preempt_disable();
 	for (;;) {
 		struct printk_log *msg;
 		size_t ext_len = 0;
@@ -2461,8 +2471,11 @@ void console_unlock(void)
 		do_handoff = should_handoff_printing(printing_start_ts);
 		printk_safe_exit_irqrestore(flags);
 
-		if (!do_handoff && do_cond_resched)
+		if (!do_handoff && do_cond_resched) {
+			preempt_enable();
 			cond_resched();
+			preempt_disable();
+		}
 	}
 	console_locked = 0;
 
@@ -2473,6 +2486,7 @@ void console_unlock(void)
 	raw_spin_unlock(&logbuf_lock);
 
 	up_console_sem();
+	preempt_enable();
 
 	/*
 	 * Someone could have filled up the buffer again, so re-check
-- 
2.15.1