Message-Id: <20140506163714.058937e8d6ec17caca406fb7@linux-foundation.org>
Date:	Tue, 6 May 2014 16:37:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>, pmladek@...e.cz,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH] printk: Remove separate printk_sched buffers and use
 printk buf instead

On Tue, 6 May 2014 07:04:46 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 6 May 2014 11:45:57 +0200
> Jan Kara <jack@...e.cz> wrote:
> 
> > On Mon 05-05-14 19:18:46, Steven Rostedt wrote:
> > > 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@...dmis.org>
> >   Andrew already has this patch in -mm tree AFAIK...
> 
> Ah, and somebody told me that it wasn't in linux-next. But looking at
> it now, it seems to be.
> 

What I have in -next is very different from this version of the patch. 
What's happening?


From: Steven Rostedt <rostedt@...dmis.org>
Subject: printk: remove separate printk_sched buffers and use printk buf instead

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@...dmis.org>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 kernel/printk/printk.c |   47 ++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff -puN kernel/printk/printk.c~printk-remove-separate-printk_sched-buffers-and-use-printk-buf-instead kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-remove-separate-printk_sched-buffers-and-use-printk-buf-instead
+++ a/kernel/printk/printk.c
@@ -68,6 +68,9 @@ int console_printk[4] = {
 	DEFAULT_CONSOLE_LOGLEVEL,	/* default_console_loglevel */
 };
 
+/* Deferred messaged from sched code are marked by this special level */
+#define SCHED_MESSAGE_LOGLEVEL -2
+
 /*
  * Low level drivers may need that to know if they can schedule in
  * their unblank() callback or not. So let's export it.
@@ -229,7 +232,9 @@ struct printk_log {
 };
 
 /*
- * The logbuf_lock protects kmsg buffer, indices, counters.
+ * The logbuf_lock protects kmsg buffer, indices, counters.  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);
 
@@ -1577,14 +1582,19 @@ asmlinkage int vprintk_emit(int facility
 	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;
 	/* cpu currently holding logbuf_lock in this function */
 	static volatile unsigned int logbuf_cpu = UINT_MAX;
 
+	if (level == SCHED_MESSAGE_LOGLEVEL) {
+		level = -1;
+		in_sched = true;
+	}
 
 	boot_delay_msec(level);
 	printk_delay();
@@ -1631,7 +1641,12 @@ asmlinkage int vprintk_emit(int facility
 	 * 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') {
@@ -1713,6 +1728,10 @@ asmlinkage int vprintk_emit(int facility
 	lockdep_on();
 	local_irq_restore(flags);
 
+	/* If called from the scheduler, we can not call up(). */
+	if (in_sched)
+		return printed_len;
+
 	/*
 	 * Disable preemption to avoid being preempted while holding
 	 * console_sem which would prevent anyone from printing to console
@@ -2531,21 +2550,19 @@ late_initcall(printk_late_init);
 /*
  * Delayed printk version, for scheduler-internal messages:
  */
-#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 trylock fails, someone else is doing the printing */
+		if (console_trylock())
+			console_unlock();
 	}
 
 	if (pending & PRINTK_PENDING_WAKEUP)
@@ -2569,21 +2586,15 @@ void wake_up_klogd(void)
 
 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, SCHED_MESSAGE_LOGLEVEL, NULL, 0, fmt, args);
 	va_end(args);
 
-	__this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
+	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 	irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
-	local_irq_restore(flags);
 
 	return r;
 }
_
--
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