[PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy

[Sergey Senozhatsky]

CON_PRINTBUFFER console registration requires us to do several
preparation steps:
- Rollback console_seq to replay logbuf messages which were already
  seen on other consoles;
- Set exclusive_console flag so console_unlock() will ->write() logbuf
  messages only to the exclusive_console driver.

The way we do it, however, is a bit racy

	logbuf_lock_irqsave(flags);
	console_seq = syslog_seq;
	console_idx = syslog_idx;
	logbuf_unlock_irqrestore(flags);
						<< preemption enabled
						<< irqs enabled
	exclusive_console = newcon;
	console_unlock();

We rollback console_seq under logbuf_lock with IRQs disabled, but
we set exclusive_console with local IRQs enabled and logbuf unlocked.
If the system oops-es or panic-s before we set exclusive_console - and
given that we have IRQs and preemption enabled there is such a
possibility - we will re-play all logbuf messages to every registered
console, which may be a bit annoying and time consuming.

Move exclusive_console assignment to the same IRQs-disabled and
logbuf_lock-protected section where we rollback console_seq.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---

v2: added comment (Petr)

 kernel/printk/printk.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 308497194bd4..24a81ba9ec77 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2742,14 +2742,18 @@ void register_console(struct console *newcon)
 		logbuf_lock_irqsave(flags);
 		console_seq = syslog_seq;
 		console_idx = syslog_idx;
-		logbuf_unlock_irqrestore(flags);
 		/*
 		 * We're about to replay the log buffer.  Only do this to the
 		 * just-registered console to avoid excessive message spam to
 		 * the already-registered consoles.
+		 *
+		 * Set exclusive_console with disabled interrupts to reduce
+		 * race window with eventual console_flush_on_panic() that
+		 * ignores console_lock.
 		 */
 		exclusive_console = newcon;
 		exclusive_console_stop_seq = console_seq;
+		logbuf_unlock_irqrestore(flags);
 	}
 	console_unlock();
 	console_sysfs_notify();
-- 
2.19.0