From 0a64ce6e5442bbd96cbe9057d9ba1edab244f25b Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Tue, 30 Aug 2022 16:50:04 +0200 Subject: [PATCH] kernel/panic: Drop unblank_screen call MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit console_unblank() does this too (called in both places right after), and with a lot more confidence inspiring approach to locking. Reconstructing this story is very strange: In b61312d353da ("oops handling: ensure that any oops is flushed to the mtdoops console") it is claimed that a printk(" "); flushed out the console buffer, which was removed in e3e8a75d2acf ("[PATCH] Extract and use wake_up_klogd()"). In todays kernels this is done way earlier in console_flush_on_panic with some really nasty tricks. I didn't bother to fully reconstruct this all, least because the call to bust_spinlock(0); gets moved every few years, depending upon how the wind blows (or well, who screamed loudest about the various issue each call site caused). Before that commit the only calls to console_unblank() where in s390 arch code. The other side here is the console->unblank callback, which was introduced in 2.1.31 for the vt driver. Which predates the console_unblank() function by a lot, which was added (without users) in 2.4.14.3. So pretty much impossible to guess at any motivation here. Also afaict the vt driver is the only (and always was the only) console driver implementing the unblank callback, so no idea why a call to console_unblank() was added for the mtdooops driver - the action actually flushing out the console buffers is done from console_unlock() only. Note that as prep for the s390 users the locking was adjusted in 2.5.22 (I couldn't figure out how to properly reference the BK commit from the historical git trees) from a normal semaphore to a trylock. Note that a copy of the direct unblank_screen() call was added to panic() in c7c3f05e341a ("panic: avoid deadlocks in re-entrant console drivers"), which partially inlined the bust_spinlocks(0); call. Long story short, I have no idea why the direct call to unblank_screen survived for so long (the infrastructure to do it properly existed for years), nor why it wasn't removed when the console_unblank() call was finally added. But it makes a ton more sense to finally do that than not - it's just better encapsulation to go through the console functions instead of doing a direct call, so let's dare. Plus it really does not make much sense to call the only unblank implementation there is twice, once without, and once with appropriate locking. Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: Daniel Vetter Cc: "Ilpo Järvinen" Cc: Tetsuo Handa Cc: Xuezhi Zhang Cc: Yangxi Xiang Cc: nick black Cc: Petr Mladek Cc: Andrew Morton Cc: Luis Chamberlain Cc: "Guilherme G. Piccoli" Cc: Marco Elver Cc: John Ogness Cc: Sebastian Andrzej Siewior Cc: David Gow Cc: tangmeng Cc: Tiezhu Yang Cc: Chris Wilson Reviewed-by: Petr Mladek Acked-by: Sebastian Andrzej Siewior Signed-off-by: Daniel Vetter Signed-off-by: Andrew Morton Link: https://lore.kernel.org/r/20220830145004.430545-1-daniel.vetter@ffwll.ch Signed-off-by: Greg Kroah-Hartman --- drivers/tty/vt/vt.c | 3 ++- include/linux/vt_kern.h | 1 - kernel/panic.c | 3 --- lib/bust_spinlocks.c | 3 --- 4 files changed, 2 insertions(+), 8 deletions(-) diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 4d29e4a..2cb515d 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -154,6 +154,7 @@ static void console_callback(struct work_struct *ignored); static void con_driver_unregister_callback(struct work_struct *ignored); static void blank_screen_t(struct timer_list *unused); static void set_palette(struct vc_data *vc); +static void unblank_screen(void); #define vt_get_kmsg_redirect() vt_kmsg_redirect(-1) @@ -4448,7 +4449,7 @@ EXPORT_SYMBOL(do_unblank_screen); * call it with 1 as an argument and so force a mode restore... that may kill * X or at least garbage the screen but would also make the Oops visible... */ -void unblank_screen(void) +static void unblank_screen(void) { do_unblank_screen(0); } diff --git a/include/linux/vt_kern.h b/include/linux/vt_kern.h index b5ab452..c1f5aeb 100644 --- a/include/linux/vt_kern.h +++ b/include/linux/vt_kern.h @@ -30,7 +30,6 @@ struct vc_data *vc_deallocate(unsigned int console); void reset_palette(struct vc_data *vc); void do_blank_screen(int entering_gfx); void do_unblank_screen(int leaving_gfx); -void unblank_screen(void); void poke_blanked_console(void); int con_font_op(struct vc_data *vc, struct console_font_op *op); int con_set_cmap(unsigned char __user *cmap); diff --git a/kernel/panic.c b/kernel/panic.c index c6eb8f8..da32320 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -329,9 +329,6 @@ void panic(const char *fmt, ...) if (_crash_kexec_post_notifiers) __crash_kexec(NULL); -#ifdef CONFIG_VT - unblank_screen(); -#endif console_unblank(); /* diff --git a/lib/bust_spinlocks.c b/lib/bust_spinlocks.c index 8be59f8..bfd53972 100644 --- a/lib/bust_spinlocks.c +++ b/lib/bust_spinlocks.c @@ -22,9 +22,6 @@ void bust_spinlocks(int yes) if (yes) { ++oops_in_progress; } else { -#ifdef CONFIG_VT - unblank_screen(); -#endif console_unblank(); if (--oops_in_progress == 0) wake_up_klogd(); -- 2.7.4