printk: release lockbuf_lock before calling console_trylock_for_printk()

There's no reason to hold lockbuf_lock when entering
console_trylock_for_printk().

The first thing this function does is to call down_trylock(console_sem)
and if that fails it immediately unlocks lockbuf_lock.  So lockbuf_lock
isn't needed for that branch.  When down_trylock() succeeds, the rest of
console_trylock() is OK without lockbuf_lock (it is called without it
from other places), and the only remaining thing in
console_trylock_for_printk() is can_use_console() call.  For that call
console_sem is enough (it iterates all consoles and checks CON_ANYTIME
flag).

So we drop logbuf_lock before entering console_trylock_for_printk() which
simplifies the code.

[akpm@linux-foundation.org: fix have_callable_console() comment]
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/printk.c

@@ -249,9 +249,6 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
 static char *log_buf = __log_buf;
 static u32 log_buf_len = __LOG_BUF_LEN;
 
-/* cpu currently holding logbuf_lock */
-static volatile unsigned int logbuf_cpu = UINT_MAX;
-
 /* human readable text of the record */
 static char *log_text(const struct printk_log *msg)
 {
@@ -1380,7 +1377,10 @@ static void zap_locks(void)
 	sema_init(&console_sem, 1);
 }
 
-/* Check if we have any console registered that can be called early in boot. */
+/*
+ * Check if we have any console that is capable of printing while cpu is
+ * booting or shutting down. Requires console_sem.
+ */
 static int have_callable_console(void)
 {
 	struct console *con;
@@ -1410,36 +1410,22 @@ static inline int can_use_console(unsigned int cpu)
  * messages from a 'printk'. Return true (and with the
  * console_lock held, and 'console_locked' set) if it
  * is successful, false otherwise.
- *
- * This gets called with the 'logbuf_lock' spinlock held and
- * interrupts disabled. It should return with 'lockbuf_lock'
- * released but interrupts still disabled.
  */
 static int console_trylock_for_printk(unsigned int cpu)
-	__releases(&logbuf_lock)
 {
-	int retval = 0, wake = 0;
-
-	if (console_trylock()) {
-		retval = 1;
-
-		/*
-		 * If we can't use the console, we need to release
-		 * the console semaphore by hand to avoid flushing
-		 * the buffer. We need to hold the console semaphore
-		 * in order to do this test safely.
-		 */
-		if (!can_use_console(cpu)) {
-			console_locked = 0;
-			wake = 1;
-			retval = 0;
-		}
-	}
-	logbuf_cpu = UINT_MAX;
-	raw_spin_unlock(&logbuf_lock);
-	if (wake)
+	if (!console_trylock())
+		return 0;
+	/*
+	 * If we can't use the console, we need to release the console
+	 * semaphore by hand to avoid flushing the buffer. We need to hold the
+	 * console semaphore in order to do this test safely.
+	 */
+	if (!can_use_console(cpu)) {
+		console_locked = 0;
 		up(&console_sem);
-	return retval;
+		return 0;
+	}
+	return 1;
 }
 
 int printk_delay_msec __read_mostly;
@@ -1572,6 +1558,9 @@ asmlinkage int vprintk_emit(int facility, int level,
 	unsigned long flags;
 	int this_cpu;
 	int printed_len = 0;
+	/* cpu currently holding logbuf_lock in this function */
+	static volatile unsigned int logbuf_cpu = UINT_MAX;
+
 
 	boot_delay_msec(level);
 	printk_delay();
@@ -1694,13 +1683,12 @@ asmlinkage int vprintk_emit(int facility, int level,
 						 dict, dictlen, text, text_len);
 	}
 
+	logbuf_cpu = UINT_MAX;
+	raw_spin_unlock(&logbuf_lock);
 	/*
 	 * Try to acquire and then immediately release the console semaphore.
 	 * The release will print out buffers and wake up /dev/kmsg and syslog()
 	 * users.
-	 *
-	 * The console_trylock_for_printk() function will release 'logbuf_lock'
-	 * regardless of whether it actually gets the console semaphore or not.
 	 */
 	if (console_trylock_for_printk(this_cpu))
 		console_unlock();