[PATCH 2/2] printk: factor out can_printk_async()

[Sergey Senozhatsky]

console_unlock() can be called directly or indirectly by a user
space process, so it can end up doing call_console_drivers() loop,
which will hold it from returning back to user-space from a syscall
for unpredictable amount of time.

Factor out can_printk_async() function, which queues an irq work and
sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
vprintk_emit() already does it, add can_printk_async() call to
console_unlock() for !PF_KTHREAD processes.
---
 kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 47a70a2..7d3a8e1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -355,6 +355,26 @@ int printk_deferred(const char *fmt, ...)
 	return r;
 }
 
+static bool can_printk_async(bool sync)
+{
+	/*
+	 * By default we print message to console asynchronously so that kernel
+	 * doesn't get stalled due to slow serial console. That can lead to
+	 * softlockups, lost interrupts, or userspace timing out under heavy
+	 * printing load.
+	 *
+	 * However we resort to synchronous printing of messages during early
+	 * boot, when oops is in progress, or when synchronous printing was
+	 * explicitely requested by kernel parameter.
+	 */
+	if (keventd_up() && !oops_in_progress && !sync) {
+		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+		return true;
+	}
+	return false;
+}
+
 /* Return log buffer address */
 char *log_buf_addr_get(void)
 {
@@ -1889,20 +1909,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	logbuf_cpu = UINT_MAX;
 	raw_spin_unlock(&logbuf_lock);
 	lockdep_on();
-	/*
-	 * By default we print message to console asynchronously so that kernel
-	 * doesn't get stalled due to slow serial console. That can lead to
-	 * softlockups, lost interrupts, or userspace timing out under heavy
-	 * printing load.
-	 *
-	 * However we resort to synchronous printing of messages during early
-	 * boot, when oops is in progress, or when synchronous printing was
-	 * explicitely requested by kernel parameter.
-	 */
-	if (keventd_up() && !oops_in_progress && !sync_print) {
-		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
-		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
-	} else
+	if (!can_printk_async(sync_print))
 		sync_print = true;
 	local_irq_restore(flags);
 
@@ -2328,6 +2335,13 @@ void console_unlock(void)
 		return;
 	}
 
+	if (!(current->flags & PF_KTHREAD) &&
+			can_printk_async(printk_sync)) {
+		console_locked = 0;
+		up_console_sem();
+		return;
+	}
+
 	/*
 	 * Console drivers are called under logbuf_lock, so
 	 * @console_may_schedule should be cleared before; however, we may
-- 
2.6.4


--4Ckj6UjgE2iN1+kY--
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