This is a note to let you know that I've just added the patch titled
printk: Correctly handle preemption in console_unlock()
to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: printk-correctly-handle-preemption-in-console_unlock.patch and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Sun Mar 18 16:55:33 CET 2018
From: Petr Mladek pmladek@suse.com Date: Fri, 24 Mar 2017 17:14:05 +0100 Subject: printk: Correctly handle preemption in console_unlock()
From: Petr Mladek pmladek@suse.com
[ Upstream commit 257ab443118bffc7fdcef38f49cf59be68a3e362 ]
Some console drivers code calls console_conditional_schedule() that looks at @console_may_schedule. The value must be cleared when the drivers are called from console_unlock() with interrupts disabled. But rescheduling is fine when the same code is called, for example, from tty operations where the console semaphore is taken via console_lock().
This is why @console_may_schedule is cleared before calling console drivers. The original value is stored to decide if we could sleep between lines.
Now, @console_may_schedule is not cleared when we call console_trylock() and jump back to the "again" goto label. This has become a problem, since the commit 6b97a20d3a7909daa066 ("printk: set may_schedule for some of console_trylock() callers"). @console_may_schedule might get enabled now.
There is also the opposite problem. console_lock() can be called only from preemptive context. It can always enable scheduling in the console code. But console_trylock() is not able to detect it when CONFIG_PREEMPT_COUNT is disabled. Therefore we should use the original @console_may_schedule value after re-acquiring the console semaphore in console_unlock().
This patch solves both problems by moving the "again" goto label.
Alternative solution was to clear and restore the value around call_console_drivers(). Then console_conditional_schedule() could be used also inside console_unlock(). But there was a potential race with console_flush_on_panic() as reported by Sergey Senozhatsky. That function should be called only where there is only one CPU and with interrupts disabled. But better be on the safe side because stopping CPUs might fail.
Fixes: 6b97a20d3a7909 ("printk: set may_schedule for some of console_trylock() callers") Link: http://lkml.kernel.org/r/1490372045-22288-1-git-send-email-pmladek@suse.com Suggested-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Cc: Steven Rostedt rostedt@goodmis.org Cc: Peter Zijlstra peterz@infradead.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Jiri Slaby jslaby@suse.cz Cc: linux-fbdev@vger.kernel.org Cc: linux-kernel@vger.kernel.org Reviewed-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/printk/printk.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
--- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2342,7 +2342,7 @@ void console_unlock(void) }
/* - * Console drivers are called under logbuf_lock, so + * Console drivers are called with interrupts disabled, so * @console_may_schedule should be cleared before; however, we may * end up dumping a lot of lines, for example, if called from * console registration path, and should invoke cond_resched() @@ -2350,11 +2350,15 @@ void console_unlock(void) * scheduling stall on a slow console leading to RCU stall and * softlockup warnings which exacerbate the issue with more * messages practically incapacitating the system. + * + * console_trylock() is not able to detect the preemptive + * context reliably. Therefore the value must be stored before + * and cleared after the the "again" goto label. */ do_cond_resched = console_may_schedule; +again: console_may_schedule = 0;
-again: /* * We released the console_sem lock, so we need to recheck if * cpu is online and (if not) is there at least one CON_ANYTIME
Patches currently in stable-queue which might be from pmladek@suse.com are
queue-4.9/printk-correctly-handle-preemption-in-console_unlock.patch queue-4.9/braille-console-fix-value-returned-by-_braille_console_setup.patch