From b74dbbe19b2551418509b04a5095d65b494e6e28 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Tue, 30 Aug 2022 16:49:45 +0200 Subject: tty/vt: Add console_lock check to vt_console_print() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit I'm scratching my head why we have this printing_lock. Digging through historical git trees shows that: - Added in 1.1.73, and I found absolutely no reason why. - Converted to atomic bitops in 2.1.125pre2, I guess as part of SMP enabling/bugfixes. - Converted to a proper spinlock in b0940003f25d ("vt: bitlock fix") because the hand-rolled atomic version lacked necessary memory barriers. Digging around in lore for that time period did also not shed further light. The only reason I think this might still be relevant today is that (to my understanding at least, ymmv) during an oops we might be printing without console_lock held. See console_flush_on_panic() and the comments in there - we flush out the console buffers irrespective of whether we managed to acquire the right locks. The strange thing is that this reason is fairly recent, because the console flushing was historically done without oops_in_progress set. This only changed in c7c3f05e341a ("panic: avoid deadlocks in re-entrant console drivers"), which removed the call to bust_spinlocks(0) (which decrements oops_in_progress again) before flushing out the console (which back then was open coded as a console_trylock/unlock pair). Note that this entire mess should be properly fixed in the printk/console layer, and not inflicted on each implementation. For now just document what's going on and check that in all other cases callers obey the locking rules. v2: WARN_CONSOLE_UNLOCKED already checks for oops_in_progress (something else that should be fixed I guess), hence remove the open-coded check I've had. Signed-off-by: Daniel Vetter Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: "Ilpo Järvinen" Cc: Daniel Vetter Cc: Xuezhi Zhang Cc: Yangxi Xiang Cc: Tetsuo Handa Cc: nick black Cc: Petr Mladek Cc: Sergey Senozhatsky Cc: Steven Rostedt Cc: John Ogness Reviewed-by: Sam Ravnborg Link: https://lore.kernel.org/r/20220830144945.430528-1-daniel.vetter@ffwll.ch Signed-off-by: Greg Kroah-Hartman --- drivers/tty/vt/vt.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'drivers/tty/vt') diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 2cb515dbda53..08498fcf080a 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -3084,7 +3084,9 @@ static void vt_console_print(struct console *co, const char *b, unsigned count) ushort start_x, cnt; int kmsg_console; - /* console busy or not yet initialized */ + WARN_CONSOLE_UNLOCKED(); + + /* this protects against concurrent oops only */ if (!spin_trylock(&printing_lock)) return; -- cgit v1.2.3