linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Date:	Fri, 11 Dec 2015 00:24:34 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Jan Kara <jack@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Srinivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long

On (12/10/15 23:52), Sergey Senozhatsky wrote:
> 
> I think we better use raw_spin_lock as a print_lock; and, apart from that,
> seems that we don't re-init in zap_lock(). So I ended up with the following
> patch on top of yours (to be folded):
> 
> - use raw_spin_lock
> - do not forget to re-init `print_lock' in zap_locks()

while we are on this, what do you guys think?


CPU1                                        CPU2
console_unlock()
  call_console_drivers()
    con->write()
       ... spin_lock ... uart, etc          panic

zap_lock() will raw_spin_lock_init(&logbuf_lock) and
sema_init(&console_sem, 1), but we still have `spin_lock'
held by con->write().

so the `panic' flush or print out will see con->write() being already
blocked.


===8<====

We do zap_lock() in printk to make a panic print out possible, but we
can end up having a locked serial console - e.g. panic has occurred
whilst CPUx was in con->write(), which takes some internal locks, thus
call_console_drivers() will perform con->write() on an already locked
console.

Try to reset() every console in zap_lock() via console specific
->reset() call.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 include/linux/console.h | 1 +
 kernel/printk/printk.c  | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index bd19434..1cb8f72 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -125,6 +125,7 @@ struct console {
 	void	(*unblank)(void);
 	int	(*setup)(struct console *, char *);
 	int	(*match)(struct console *, char *name, int idx, char *options);
+	void	(*reset)(struct console *);
 	short	flags;
 	short	index;
 	int	cflag;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f4a9565..ad172c4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1580,6 +1580,7 @@ static void call_console_drivers(int level,
  */
 static void zap_locks(void)
 {
+	struct console *c;
 	static unsigned long oops_timestamp;
 
 	if (time_after_eq(jiffies, oops_timestamp) &&
@@ -1589,6 +1590,11 @@ static void zap_locks(void)
 	oops_timestamp = jiffies;
 
 	debug_locks_off();
+
+	for_each_console(c)
+		if ((c->flags & CON_ENABLED) && c->reset)
+			c->reset(c);
+
 	/* If a crash is occurring, make sure we can't deadlock */
 	raw_spin_lock_init(&logbuf_lock);
 	/* And make sure that we print immediately */
-- 
2.6.3

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