Commit dbdda842 authored by Steven Rostedt (VMware)'s avatar Steven Rostedt (VMware) Committed by Petr Mladek
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printk: Add console owner and waiter logic to load balance console writes

This patch implements what I discussed in Kernel Summit. I added
lockdep annotation (hopefully correctly), and it hasn't had any splats
(since I fixed some bugs in the first iterations). It did catch
problems when I had the owner covering too much. But now that the owner
is only set when actively calling the consoles, lockdep has stayed

Here's the design again:

I added a "console_owner" which is set to a task that is actively
writing to the consoles. It is *not* the same as the owner of the
console_lock. It is only set when doing the calls to the console
functions. It is protected by a console_owner_lock which is a raw spin

There is a console_waiter. This is set when there is an active console
owner that is not current, and waiter is not set. This too is protected
by console_owner_lock.

In printk() when it tries to write to the consoles, we have:

	if (console_trylock())

Now I added an else, which will check if there is an active owner, and
no current waiter. If that is the case, then console_waiter is set, and
the task goes into a spin until it is no longer set.

When the active console owner finishes writing the current message to
the consoles, it grabs the console_owner_lock and sees if there is a
waiter, and clears console_owner.

If there is a waiter, then it breaks out of the loop, clears the waiter
flag (because that will release the waiter from its spin), and exits.
Note, it does *not* release the console semaphore. Because it is a
semaphore, there is no owner. Another task may release it. This means
that the waiter is guaranteed to be the new console owner! Which it

Then the waiter calls console_unlock() and continues to write to the

If another task comes along and does a printk() it too can become the
new waiter, and we wash rinse and repeat!

By Petr Mladek about possible new deadlocks:

The thing is that we move console_sem only to printk() call
that normally calls console_unlock() as well. It means that
the transferred owner should not bring new type of dependencies.
As Steven said somewhere: "If there is a deadlock, it was
there even before."

We could look at it from this side. The possible deadlock would
look like:

CPU0                            CPU1


  console_owner = current;

				    spin = true;
				    while (...)


This would be a deadlock. CPU0 would wait for the lock A.
While CPU1 would own the lockA and would wait for CPU0
to finish calling the console drivers and pass the console_sem

But if the above is true than the following scenario was
already possible before:



By other words, this deadlock was there even before. Such
deadlocks are prevented by using printk_deferred() in
the sections guarded by the lock A.

By Steven Rostedt:

To demonstrate the issue, this module has been shown to lock up a
system with 4 CPUs and a slow console (like a serial console). It is
also able to lock up a 8 CPU system with only a fast (VGA) console, by
passing in "loops=100". The changes in this commit prevent this module
from locking up the system.

 #include <linux/module.h>
 #include <linux/delay.h>
 #include <linux/sched.h>
 #include <linux/mutex.h>
 #include <linux/workqueue.h>
 #include <linux/hrtimer.h>

 static bool stop_testing;
 static unsigned int loops = 1;

 static void preempt_printk_workfn(struct work_struct *work)
 	int i;

 	while (!READ_ONCE(stop_testing)) {
 		for (i = 0; i < loops && !READ_ONCE(stop_testing); i++) {
 			pr_emerg("%5d%-75s\n", smp_processor_id(),
 				 " XXX NOPREEMPT");

 static struct work_struct __percpu *works;

 static void finish(void)
 	int cpu;

 	WRITE_ONCE(stop_testing, true);
 		flush_work(per_cpu_ptr(works, cpu));

 static int __init test_init(void)
 	int cpu;

 	works = alloc_percpu(struct work_struct);
 	if (!works)
 		return -ENOMEM;

 	 * This is just a test module. This will break if you
 	 * do any CPU hot plugging between loading and
 	 * unloading the module.

 	for_each_online_cpu(cpu) {
 		struct work_struct *work = per_cpu_ptr(works, cpu);

 		INIT_WORK(work, &preempt_printk_workfn);
 		schedule_work_on(cpu, work);

 	return 0;

 static void __exit test_exit(void)

 module_param(loops, uint, 0);


Cc: Cong Wang <>
Cc: Dave Hansen <>
Cc: Johannes Weiner <>
Cc: Mel Gorman <>
Cc: Michal Hocko <>
Cc: Vlastimil Babka <>
Cc: Peter Zijlstra <>
Cc: Linus Torvalds <>
Cc: Jan Kara <>
Cc: Mathieu Desnoyers <>
Cc: Tetsuo Handa <>
Cc: Byungchul Park <>
Cc: Tejun Heo <>
Cc: Pavel Machek <>
Signed-off-by: default avatarSteven Rostedt (VMware) <>
[ Commit message about possible deadlocks]
Acked-by: default avatarSergey Senozhatsky <>
Signed-off-by: default avatarPetr Mladek <>
parent 11ca75d2
......@@ -86,8 +86,15 @@ EXPORT_SYMBOL_GPL(console_drivers);
static struct lockdep_map console_lock_dep_map = {
.name = "console_lock"
static struct lockdep_map console_owner_dep_map = {
.name = "console_owner"
static DEFINE_RAW_SPINLOCK(console_owner_lock);
static struct task_struct *console_owner;
static bool console_waiter;
enum devkmsg_log_bits {
......@@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility, int level,
* semaphore. The release will print out buffers and wake up
* /dev/kmsg and syslog() users.
if (console_trylock())
if (console_trylock()) {
} else {
struct task_struct *owner = NULL;
bool waiter;
bool spin = false;
owner = READ_ONCE(console_owner);
waiter = READ_ONCE(console_waiter);
if (!waiter && owner && owner != current) {
WRITE_ONCE(console_waiter, true);
spin = true;
* If there is an active printk() writing to the
* consoles, instead of having it write our data too,
* see if we can offload that load from the active
* printer, and do some printing ourselves.
* Go into a spin only if there isn't already a waiter
* spinning, and there is an active printer, and
* that active printer isn't us (recursive printk?).
if (spin) {
/* We spin waiting for the owner to release us */
spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
/* Owner will clear console_waiter on hand off */
while (READ_ONCE(console_waiter))
spin_release(&console_owner_dep_map, 1, _THIS_IP_);
* The owner passed the console lock to us.
* Since we did not spin on console lock, annotate
* this as a trylock. Otherwise lockdep will
* complain.
mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
return printed_len;
......@@ -2141,6 +2196,7 @@ void console_unlock(void)
static u64 seen_seq;
unsigned long flags;
bool wake_klogd = false;
bool waiter = false;
bool do_cond_resched, retry;
if (console_suspended) {
......@@ -2229,14 +2285,64 @@ void console_unlock(void)
* While actively printing out messages, if another printk()
* were to occur on another CPU, it may wait for this one to
* finish. This task can not be preempted if there is a
* waiter waiting to take over.
console_owner = current;
/* The waiter may spin on us after setting console_owner */
spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
stop_critical_timings(); /* don't trace print latency */
call_console_drivers(ext_text, ext_len, text, len);
waiter = READ_ONCE(console_waiter);
console_owner = NULL;
* If there is a waiter waiting for us, then pass the
* rest of the work load over to that waiter.
if (waiter)
/* There was no waiter, and nothing will spin on us here */
spin_release(&console_owner_dep_map, 1, _THIS_IP_);
if (do_cond_resched)
* If there is an active waiter waiting on the console_lock.
* Pass off the printing to the waiter, and the waiter
* will continue printing on its CPU, and when all writing
* has finished, the last printer will wake up klogd.
if (waiter) {
WRITE_ONCE(console_waiter, false);
/* The waiter is now free to continue */
spin_release(&console_owner_dep_map, 1, _THIS_IP_);
* Hand off console_lock to waiter. The waiter will perform
* the up(). After this, the waiter is the console_lock owner.
mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
/* Note, if waiter is set, logbuf_lock is not held */
console_locked = 0;
/* Release the exclusive_console once it is used */
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