Re: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes

[Steven Rostedt]

On Wed, 10 Jan 2018 14:24:17 +0100
Petr Mladek <pmladek@xxxxxxxx> wrote:

> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> 
> 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
> quiet.
> 
> 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
> lock.
> 
> 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())
> 		console_unlock();
> 
> 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
> becomes.
> 
> Then the waiter calls console_unlock() and continues to write to the
> consoles.
> 
> 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_unlock()
> 
>   console_owner = current;
> 
> 				spin_lockA()
> 				  printk()
> 				    spin = true;
> 				    while (...)
> 
>     call_console_drivers()
>       spin_lockA()
> 
> 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
> owner.
> 
> But if the above is true than the following scenario was
> already possible before:
> 
> CPU0
> 
> spin_lockA()
>   printk()
>     console_unlock()
>       call_console_drivers()
> 	spin_lockA()
> 
> 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.

Petr,

Please add this here:

====

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++) {
			preempt_disable();
			pr_emerg("%5d%-75s\n", smp_processor_id(),
				 " XXX NOPREEMPT");
			preempt_enable();
		}
		msleep(1);
	}
}

static struct work_struct __percpu *works;

static void finish(void)
{
	int cpu;

	WRITE_ONCE(stop_testing, true);
	for_each_online_cpu(cpu)
		flush_work(per_cpu_ptr(works, cpu));
	free_percpu(works);
}

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)
{
	finish();
}

module_param(loops, uint, 0);
module_init(test_init);
module_exit(test_exit);
MODULE_LICENSE("GPL");
====

Hmm, how does one have git commit not remove the C preprocessor at the
start of the module?

-- Steve

> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> [pmladek@xxxxxxxx: Commit message about possible deadlocks]
> ---