printk: CON_PRINTBUFFER console registration is a bit racy

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.

Link: http://lkml.kernel.org/r/20180928095304.9972-1-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index cfaa211a8b5480c76b63735354029f0813900292..c5b568c2d167a15af3a9e51caecde89a3140f74c 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2703,14 +2703,18 @@ 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();