Patch "printk: do cond_resched() between lines while outputting to consoles" has been added to the 4.4-stable tree

PATCH [email protected] 1 participant 1 message Feb 13, 2016

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Stable branch Printk Console

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2016-02-13 22:46 UTC #0 lore raw

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 8d91f8b15361dfb438ab6eb3b319e2ded43458ff Mon Sep 17 00:00:00 2001

From: Tejun Heo <[email protected]>
Date: Fri, 15 Jan 2016 16:58:24 -0800
Subject: printk: do cond_resched() between lines while outputting to consoles

From: Tejun Heo <[email protected]>

commit 8d91f8b15361dfb438ab6eb3b319e2ded43458ff upstream.

@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 Tejun Heo <[email protected]>

Reported-by Calvin Owens <[email protected]>

Acked-by Jan Kara <[email protected]>

Cc Dave Jones <[email protected]>

Cc Kyle McMartin <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

Signed-off-by Linus Torvalds <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 include/linux/console.h |    1 +
 kernel/panic.c          |    3 +--
 kernel/printk/printk.c  |   35 ++++++++++++++++++++++++++++++++++-
 3 files changed, 36 insertions(+), 3 deletions(-)

include/linux/console.h +1 -0

@@ -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

@@ -157,8 +157,7 @@ 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

@@ -2233,13 +2233,24 @@ 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 */

@@ -2311,6 +2322,9 @@ skip:
 		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;

@@ -2378,6 +2392,25 @@ 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
  */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch