[patch 127/146] printk: do cond_resched() between lines while outputting to consoles

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From: Tejun Heo <tj@xxxxxxxxxx>
Subject: 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: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Calvin Owens <calvinowens@xxxxxx>
Acked-by: Jan Kara <jack@xxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

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

diff -puN include/linux/console.h~printk-do-cond_resched-between-lines-while-outputting-to-consoles include/linux/console.h
--- a/include/linux/console.h~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/include/linux/console.h
@@ -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 *);
diff -puN kernel/panic.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/panic.c
--- a/kernel/panic.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/kernel/panic.c
@@ -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;
diff -puN kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
+++ a/kernel/printk/printk.c
@@ -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 @@ 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;
 
@@ -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
  */
_
--
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