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Commit 8d91f8b1 authored by Tejun Heo's avatar Tejun Heo Committed by Linus Torvalds
Browse files

printk: do cond_resched() between lines while outputting to consoles 



@console_may_schedule tracks whether console_sem was acquired through
lock or trylock.  If the former, we're inside a sleepable context and
console_conditional_schedule() performs cond_resched().  This allows
console drivers which use console_lock for synchronization to yield
while performing time-consuming operations such as scrolling.

However, the actual console outputting is performed while holding
irq-safe logbuf_lock, so console_unlock() clears @console_may_schedule
before starting outputting lines.  Also, only a few drivers call
console_conditional_schedule() to begin with.  This means that when a
lot of lines need to be output by console_unlock(), for example on a
console registration, the task doing console_unlock() may not yield for
a long time on a non-preemptible kernel.

If this happens with a slow console devices, for example a serial
console, the outputting task may occupy the cpu for a very long time.
Long enough to trigger softlockup and/or RCU stall warnings, which in
turn pile more messages, sometimes enough to trigger the next cycle of
warnings incapacitating the system.

Fix it by making console_unlock() insert cond_resched() between lines if
@console_may_schedule.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarCalvin Owens <calvinowens@fb.com>
Acked-by: default avatarJan Kara <jack@suse.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Kyle McMartin <kyle@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 81cc26f2

include/linux/console.h

+1 −0
Original line number Diff line number Diff line
@@ -150,6 +150,7 @@ extern int console_trylock(void); 
extern void console_unlock(void);

extern void console_conditional_schedule(void);

extern void console_unblank(void);

extern void console_flush_on_panic(void);

extern struct tty_driver *console_device(int *);

extern void console_stop(struct console *);

extern void console_start(struct console *);

kernel/panic.c

+1 −2
Original line number Diff line number Diff line
@@ -180,8 +180,7 @@ void panic(const char *fmt, ...) 
	 * panic() is not being callled from OOPS.

	 */

	debug_locks_off();

	console_trylock();

	console_unlock();

	console_flush_on_panic();



	if (!panic_blink)

		panic_blink = no_blink;

kernel/printk/printk.c

+34 −1
Original line number Diff line number Diff line
@@ -2234,13 +2234,24 @@ void console_unlock(void) 
	static u64 seen_seq;

	unsigned long flags;

	bool wake_klogd = false;

	bool retry;

	bool do_cond_resched, retry;



	if (console_suspended) {

		up_console_sem();

		return;

	}



	/*

	 * Console drivers are called under logbuf_lock, 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()

	 * between lines if allowable.  Not doing so can cause a very long

	 * scheduling stall on a slow console leading to RCU stall and

	 * softlockup warnings which exacerbate the issue with more

	 * messages practically incapacitating the system.

	 */

	do_cond_resched = console_may_schedule;

	console_may_schedule = 0;



	/* flush buffered message fragment immediately to console */
@@ -2312,6 +2323,9 @@ void console_unlock(void) 
		call_console_drivers(level, ext_text, ext_len, text, len);

		start_critical_timings();

		local_irq_restore(flags);



		if (do_cond_resched)

			cond_resched();

	}

	console_locked = 0;
@@ -2379,6 +2393,25 @@ void console_unblank(void) 
	console_unlock();

}



/**

 * console_flush_on_panic - flush console content on panic

 *

 * Immediately output all pending messages no matter what.

 */

void console_flush_on_panic(void)

{

	/*

	 * If someone else is holding the console lock, trylock will fail

	 * and may_schedule may be set.  Ignore and proceed to unlock so

	 * that messages are flushed out.  As this can be called from any

	 * context and we don't want to get preempted while flushing,

	 * ensure may_schedule is cleared.

	 */

	console_trylock();

	console_may_schedule = 0;

	console_unlock();

}



/*

 * Return the console tty driver structure and its associated index

 */