[PATCH] printk: Remove separate printk_sched buffers and use printk buf instead

[Steven Rostedt]

To prevent deadlocks with doing a printk inside the scheduler,
printk_sched() was created. The issue is that printk has a console_sem
that it can grab and release. The release does a wake up if there's a
task pending on the sem, and this wake up grabs the rq locks that is
held in the scheduler. This leads to a possible deadlock if the wake up
uses the same rq as the one with the rq lock held already.

What printk_sched() does is to save the printk write in a per cpu buffer
and sets the PRINTK_PENDING_SCHED flag. On a timer tick, if this flag is
set, the printk() is done against the buffer.

There's a couple of issues with this approach.

1) If two printk_sched()s are called before the tick, the second one
will overwrite the first one.

2) The temporary buffer is 512 bytes and is per cpu. This is a quite a
bit of space wasted for something that is seldom used.

In order to remove this, the printk_sched() can use the printk buffer
instead, and delay the console_trylock()/console_unlock() to the queued
work.

Because printk_sched() would then be taking the logbuf_lock, the
logbuf_lock must not be held while doing anything that may call into the
scheduler functions, which includes wake ups. Unfortunately, printk()
also has a console_sem that it uses, and on release, the
up(&console_sem) may do a wake up of any pending waiters. This must be
avoided while holding the logbuf_lock.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
This version has been forward ported to the 3.15-rc releases.
---
 kernel/printk/printk.c |   87 +++++++++++++++++++++++++++++++------------------
 1 file changed, 56 insertions(+), 31 deletions(-)

Index: linux-trace.git/kernel/printk/printk.c
===================================================================
--- linux-trace.git.orig/kernel/printk/printk.c	2014-05-05 16:46:17.280815365 -0400
+++ linux-trace.git/kernel/printk/printk.c	2014-05-05 16:52:23.398378897 -0400
@@ -208,6 +208,9 @@
 /*
  * The logbuf_lock protects kmsg buffer, indices, counters. It is also
  * used in interesting ways to provide interlocking in console_unlock();
+ * This can be taken within the scheduler's rq lock. It must be released
+ * before calling console_unlock() or anything else that might wake up
+ * a process.
  */
 static DEFINE_RAW_SPINLOCK(logbuf_lock);
 
@@ -1338,27 +1341,43 @@
  * interrupts disabled. It should return with 'lockbuf_lock'
  * released but interrupts still disabled.
  */
-static int console_trylock_for_printk(unsigned int cpu)
+static int console_trylock_for_printk(unsigned int cpu, bool in_sched)
 	__releases(&logbuf_lock)
 {
 	int retval = 0, wake = 0;
 
-	if (console_trylock()) {
-		retval = 1;
+	/* if called from the scheduler, we can not call up() */
+	if (in_sched)
+		goto out;
 
-		/*
-		 * If we can't use the console, we need to release
-		 * the console semaphore by hand to avoid flushing
-		 * the buffer. We need to hold the console semaphore
-		 * in order to do this test safely.
-		 */
-		if (!can_use_console(cpu)) {
-			console_locked = 0;
-			wake = 1;
-			retval = 0;
-		}
+	if (down_trylock(&console_sem))
+		goto out;
+
+	/*
+	 * If we can't use the console, we need to release
+	 * the console semaphore by hand to avoid flushing
+	 * the buffer. We need to hold the console semaphore
+	 * in order to do this test safely.
+	 */
+	if (console_suspended || !can_use_console(cpu)) {
+		wake = 1;
+		goto out;
 	}
+
+	/* console is now locked */
+
+	console_locked = 1;
+	console_may_schedule = 0;
+	mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_);
+
+	retval = 1;
+
+out:
 	logbuf_cpu = UINT_MAX;
+	/*
+	 * The logbuf_lock must not be held when doing a wake up,
+	 * which the up(&console_sem) can do.
+	 */
 	raw_spin_unlock(&logbuf_lock);
 	if (wake)
 		up(&console_sem);
@@ -1490,11 +1509,17 @@
 	static int recursion_bug;
 	static char textbuf[LOG_LINE_MAX];
 	char *text = textbuf;
-	size_t text_len;
+	size_t text_len = 0;
 	enum log_flags lflags = 0;
 	unsigned long flags;
 	int this_cpu;
 	int printed_len = 0;
+	bool in_sched = false;
+
+	if (level == -2) {
+		level = -1;
+		in_sched = true;
+	}
 
 	boot_delay_msec(level);
 	printk_delay();
@@ -1540,7 +1565,12 @@
 	 * The printf needs to come first; we need the syslog
 	 * prefix which might be passed-in as a parameter.
 	 */
-	text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
+	if (in_sched)
+		text_len = scnprintf(text, sizeof(textbuf),
+				     KERN_WARNING "[sched_delayed] ");
+
+	text_len += vscnprintf(text + text_len,
+			       sizeof(textbuf) - text_len, fmt, args);
 
 	/* mark and strip a trailing newline */
 	if (text_len && text[text_len-1] == '\n') {
@@ -1621,7 +1651,7 @@
 	 * The console_trylock_for_printk() function will release 'logbuf_lock'
 	 * regardless of whether it actually gets the console semaphore or not.
 	 */
-	if (console_trylock_for_printk(this_cpu))
+	if (console_trylock_for_printk(this_cpu, in_sched))
 		console_unlock();
 
 	lockdep_on();
@@ -2440,18 +2470,20 @@
 #define PRINTK_BUF_SIZE		512
 
 #define PRINTK_PENDING_WAKEUP	0x01
-#define PRINTK_PENDING_SCHED	0x02
+#define PRINTK_PENDING_OUTPUT	0x02
 
 static DEFINE_PER_CPU(int, printk_pending);
-static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
 
 static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
 	int pending = __this_cpu_xchg(printk_pending, 0);
 
-	if (pending & PRINTK_PENDING_SCHED) {
-		char *buf = __get_cpu_var(printk_sched_buf);
-		pr_warn("[sched_delayed] %s", buf);
+	if (pending & PRINTK_PENDING_OUTPUT) {
+		if (console_trylock())
+			console_unlock();
+		else
+			/* Try again later */
+			__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 	}
 
 	if (pending & PRINTK_PENDING_WAKEUP)
@@ -2475,21 +2507,14 @@
 
 int printk_sched(const char *fmt, ...)
 {
-	unsigned long flags;
 	va_list args;
-	char *buf;
 	int r;
 
-	local_irq_save(flags);
-	buf = __get_cpu_var(printk_sched_buf);
-
 	va_start(args, fmt);
-	r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args);
+	r = vprintk_emit(0, -2, NULL, 0, fmt, args);
 	va_end(args);
 
-	__this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
-	irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
-	local_irq_restore(flags);
+	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 
 	return r;
 }
--
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