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Commit 458df9fd authored by Steven Rostedt's avatar Steven Rostedt Committed by Linus Torvalds
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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: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 939f04be

kernel/printk/printk.c

+29 −18
Original line number Diff line number Diff line
@@ -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, int level, 
	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, int level, 
	 * 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, int level, 
	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
@@ -2532,21 +2551,19 @@ 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)
@@ -2570,21 +2587,15 @@ 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;

}