linux-kernel - [PATCH 3.10 042/180] printk: do cond_resched() between lines while outputting to consoles

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Message-Id: <1471793510-13022-43-git-send-email-w@1wt.eu>
Date:   Sun, 21 Aug 2016 17:29:32 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Tejun Heo <tj@...nel.org>, Dave Jones <davej@...emonkey.org.uk>,
        Kyle McMartin <kyle@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Chas Williams <ciwillia@...cade.com>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 042/180] printk: do cond_resched() between lines while outputting to consoles

From: Tejun Heo <tj@...nel.org>

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 <tj@...nel.org>
Reported-by: Calvin Owens <calvinowens@...com>
Acked-by: Jan Kara <jack@...e.com>
Cc: Dave Jones <davej@...emonkey.org.uk>
Cc: Kyle McMartin <kyle@...nel.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[ciwillia@...cade.com: adjust context for 3.10.y]
Signed-off-by: Chas Williams <ciwillia@...cade.com>
Signed-off-by: Willy Tarreau <w@....eu>
---
 include/linux/console.h |  1 +
 kernel/panic.c          |  3 +++
 kernel/printk.c         | 35 ++++++++++++++++++++++++++++++++++-
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 73bab0f..6877ffc 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -153,6 +153,7 @@ 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 --git a/kernel/panic.c b/kernel/panic.c
index 167ec097..d3d74c4 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -22,6 +22,7 @@
 #include <linux/sysrq.h>
 #include <linux/init.h>
 #include <linux/nmi.h>
+#include <linux/console.h>
 
 #define PANIC_TIMER_STEP 100
 #define PANIC_BLINK_SPD 18
@@ -128,6 +129,8 @@ void panic(const char *fmt, ...)
 
 	bust_spinlocks(0);
 
+	console_flush_on_panic();
+
 	if (!panic_blink)
 		panic_blink = no_blink;
 
diff --git a/kernel/printk.c b/kernel/printk.c
index fd0154a..ee8f6be 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -2033,13 +2033,24 @@ 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 */
@@ -2096,6 +2107,9 @@ skip:
 		call_console_drivers(level, text, len);
 		start_critical_timings();
 		local_irq_restore(flags);
+
+		if (do_cond_resched)
+			cond_resched();
 	}
 	console_locked = 0;
 	mutex_release(&console_lock_dep_map, 1, _RET_IP_);
@@ -2164,6 +2178,25 @@ 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
  */
-- 
2.8.0.rc2.1.gbe9624a