Re: [PATCH] printk: stop spining waiter when console resume to flush prb

[Petr Mladek <pmladek@suse.com>]

On Thu 2021-05-06 16:00:26, Luo Jiaxing wrote:
> Some threads still call printk() for printing when resume_console() is
> being executed. In practice, the printk() is executed for a period of time
> and then returned. The duration is determined by the number of prints
> cached in the prb during the suspend/resume process. At the same time,
> resume_console() returns quickly.

The last sentence is a bit misleading. resume_console() returns
quickly only when @console_owner was passed to another process.


> Base on owner/waiter machanism, the frist one who fail to lock console will
> become waiter, and start spining. When current owner finish print one
> informance, if a waiter is waitting, owner will give up and let waiter
> become a new owner. New owner need to flush the whole prb unitl prb empty
> or another new waiter come and take the job from him.
>
> So the first waiter after resume_console() will take seconds to help to

It need not to be the first waiter. The console_lock owner might be passed
several times.

But you have a point. Many messages might get accumulated when the
console was suspended and any console_owner might spend a long time
processing them. resume_console() seems to be always called in
preemptible context, so it is safe to process all messages here.


> flush prb, but driver which call printk() may be bothered by this. New
> a flag to mark resume flushing prb. When the console resume, before the
> prb is empty, stop to set a new waiter temporarily.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -287,6 +287,9 @@ EXPORT_SYMBOL(console_set_on_cmdline);
>  /* Flag: console code may call schedule() */
>  static int console_may_schedule;
>  
> +/* Flags: console flushing prb when resume */
> +static atomic_t console_resume_flush_prb = ATOMIC_INIT(0);
> +
>  enum con_msg_format_flags {
>  	MSG_FORMAT_DEFAULT	= 0,
>  	MSG_FORMAT_SYSLOG	= (1 << 0),
> @@ -1781,7 +1784,8 @@ static int console_trylock_spinning(void)
>  	raw_spin_lock(&console_owner_lock);
>  	owner = READ_ONCE(console_owner);
>  	waiter = READ_ONCE(console_waiter);
> -	if (!waiter && owner && owner != current) {
> +	if (!waiter && owner && owner != current &&
> +	    !atomic_read(&console_resume_flush_prb)) {

atomic_set()/atomic_read() do not provide any memory barriers.
IMHO, the atomic operations are not enough to serialize @console_owner
and @console_resume_flush_prb manipulation.

See below.

>  		WRITE_ONCE(console_waiter, true);
>  		spin = true;
>  	}
> @@ -2355,6 +2359,7 @@ void resume_console(void)
>  	if (!console_suspend_enabled)
>  		return;
>  	down_console_sem();
> +	atomic_set(&console_resume_flush_prb, 1);
>  	console_suspended = 0;
>  	console_unlock();
>  }
> @@ -2592,6 +2597,8 @@ void console_unlock(void)
>  	raw_spin_unlock(&logbuf_lock);
>  
>  	up_console_sem();
> +	if (atomic_read(&console_resume_flush_prb))
> +		atomic_set(&console_resume_flush_prb, 0);

This should be done under console_lock. Othwerwise,
it is not serialized at all.

Also there is one more return from console_unlock():

	if (!can_use_console()) {
		console_locked = 0;
		up_console_sem();
		return;
	}

@console_resume_flush_prb must be cleared here as well.
Otherwise, the next random console_unlock() caller will not
be allowed to pass the console lock owner.


OK, the above patch tries to tell console_trylock_spinning()
that it should ignore console_owner even when set.
@console_resume_flush_prb variable is set/read by different
processes in parallel which makes it complicated.

Instead, we should simply tell console_unlock() that it should not
set console_owner in this case. The most strightforward
way is to pass this via parameter.

Such console_unlock() might be used even on another locations
with preemptible context.


What about the following patch?


From 574e844f512c9f450e64832f09cc389bc9915f83 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Thu, 6 May 2021 12:40:56 +0200
Subject: [PATCH] printk: Prevent softlockup when resuming console

Many printk messages might get accumulated when consoles were suspended.
They are proceed when console_unlock() is called in resume_console().

The possibility to pass the console lock owner was added to reduce the risk
of softlockup when too many messages were handled in an atomic context.

Now, resume_console() is always in a preemptible context that is safe
to handle all accumulated messages. The possibility to pass the console
lock owner actually makes things worse. The new owner might be in an atomic
context and might cause softlockup when processing all messages accumulated
when the console was suspended.

Create new console_unlock_preemptible() that will not allow to pass
the console lock owner. As a result, all accumulated messages will
be proceed in the safe preemptible process.

Use it in resume_console(). But it might be used also on other locations
where console lock was used in preemptible context and many messages
might get accumulated when the process was sleeping.

Reported-by: Luo Jiaxing <luojiaxing@huawei.com>
Suggested-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/console.h |  1 +
 kernel/printk/printk.c  | 39 +++++++++++++++++++++++++++++++++------
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 20874db50bc8..0c444c6448e8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -174,6 +174,7 @@ extern struct console *console_drivers;
 extern void console_lock(void);
 extern int console_trylock(void);
 extern void console_unlock(void);
+extern void console_unlock_preemptible(void);
 extern void console_conditional_schedule(void);
 extern void console_unblank(void);
 extern void console_flush_on_panic(enum con_flush_mode mode);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 421c35571797..a7e94c898646 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2425,7 +2425,7 @@ void resume_console(void)
 		return;
 	down_console_sem();
 	console_suspended = 0;
-	console_unlock();
+	console_unlock_preemptible();
 }
 
 /**
@@ -2534,10 +2534,8 @@ static inline int can_use_console(void)
  * the output prior to releasing the lock.
  *
  * If there is output waiting, we wake /dev/kmsg and syslog() users.
- *
- * console_unlock(); may be called from any context.
  */
-void console_unlock(void)
+void __console_unlock(bool spinning_enabled)
 {
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[CONSOLE_LOG_MAX];
@@ -2546,6 +2544,7 @@ void console_unlock(void)
 	struct printk_info info;
 	struct printk_record r;
 
+
 	if (console_suspended) {
 		up_console_sem();
 		return;
@@ -2637,13 +2636,15 @@ void console_unlock(void)
 		 * finish. This task can not be preempted if there is a
 		 * waiter waiting to take over.
 		 */
-		console_lock_spinning_enable();
+		if (spinning_enabled)
+			console_lock_spinning_enable();
 
 		stop_critical_timings();	/* don't trace print latency */
 		call_console_drivers(ext_text, ext_len, text, len);
 		start_critical_timings();
 
-		if (console_lock_spinning_disable_and_check()) {
+		if (spinning_enabled &&
+		    console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
 			return;
 		}
@@ -2670,8 +2671,34 @@ void console_unlock(void)
 	if (retry && console_trylock())
 		goto again;
 }
+
+/*
+ * Classic console_unlock() that might be called in any context.
+ *
+ * It allows to pass the console lock owner when processing the buffered
+ * messages. It helps to prevent soft lockups in an atomic context.
+ */
+void console_unlock()
+{
+	__console_unlock(true);
+}
 EXPORT_SYMBOL(console_unlock);
 
+/*
+ * Variant of the console unlock that can be called only in preemptible
+ * context.
+ *
+ * All messages are processed in this safe context. It helps to prevent
+ * softlockups when the console lock owner was passed to an atomic context.
+ */
+void console_unlock_preemptible()
+{
+	lockdep_assert_preemption_enabled();
+
+	__console_unlock(false);
+}
+EXPORT_SYMBOL(console_unlock_preemptible);
+
 /**
  * console_conditional_schedule - yield the CPU if required
  *
-- 
2.26.2