Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure

From: Sergey Senozhatsky <[email-masked]>
Date: Wed, 20 Nov 2019 10:30:05 +0900

On (19/11/19 10:41), Petr Mladek wrote:
[..]
> > > I do not like this. As a result, normal printk() will always deadlock
> > > in the scheduler code, including WARN() calls. The chance of the
> > > deadlock is small now. It happens only when there is another
> > > process waiting for console_sem.
> > 
> > Why would it *always* deadlock? If this is the case, why we don't *always*
> > deadlock doing the very same wake_up_process() from console_unlock()?
> 
> I speak about _normal_ printk() and not about printk_deferred().
> 
> wake_up_process() is called in console_unlock() only when
> sem->wait_list is not empty, see up() in kernel/locking/semaphore.c.
> printk() itself uses console_trylock() and does not wait.

Sorry, I should have spent more time on this. So here's what I should
have written instead.

console_sem is not for kthreads or kernel only. User space apps can sleep
on it. And on some setups console_sem becomes really congested, with often
times non-empty console_sem->wait_list. All because of user space, we don't
even need printk()-s. For example, when user space uses tty for I/O.

Significantly trimmed logs from my laptop (a simple patch below). Just
to show that user space sleeps on console_sem (IOW, non-empty ->wait_list).

Notice - ls, bash, journalctl, etc. All competing for console_sem.

[..]
 printk: >>> bash WOKEUP console_sem() waiter
 printk: task journalctl couldn't lock console_sem and had to schedule out
 Call Trace:
  dump_stack+0x50/0x70
  console_lock.cold+0x21/0x26
  con_write+0x28/0x50
  n_tty_write+0x12f/0x4c0
  ? wait_woken+0xa0/0xa0
  tty_write+0x1a9/0x2f0
  ? n_tty_read+0x930/0x930
  vfs_write+0xb7/0x1e0

printk: >>> ls WOKEUP console_sem() waiter
printk: task kworker/3:1 couldn't lock console_sem and had to schedule out
Workqueue: events console_callback
Call Trace:
 dump_stack+0x50/0x70
 console_lock.cold+0x21/0x26
 console_callback+0xa/0x150
 ? _cond_resched+0x15/0x50
 process_one_work+0x18b/0x2e0
 worker_thread+0x4e/0x3d0
 ? process_one_work+0x2e0/0x2e0
 kthread+0x119/0x130

printk: >>> kworker/3:1 WOKEUP console_sem() waiter
printk: task ls couldn't lock console_sem and had to schedule out
Call Trace:
 dump_stack+0x50/0x70
 console_lock.cold+0x21/0x26
 con_write+0x28/0x50
 do_output_char+0x191/0x1f0
 n_tty_write+0x17e/0x4c0
 ? __switch_to_asm+0x24/0x60
 ? wait_woken+0xa0/0xa0
 tty_write+0x1a9/0x2f0
 ? n_tty_read+0x930/0x930
 vfs_write+0xb7/0x1e0
[..]

> I believe that this is the rason why printk_sched() was added
> so late in 2012. It was more than 10 years after adding
> the semaphore into console_unlock(). IMHO, the deadlock
> was rare. Of course, it was also hard to debug but it
> would not take 10 years.

Right. I also think scheduler people do pretty nice work avoiding printk
calls under ->rq lock.

What I tried to say - it's really not that hard to have a non-empty
console_sem ->wait_list, any "wrong" printk() call from scheduler
will deadlock us, because we have something to wake_up().

///////////////////////////////////////////////////////////////////////
// A simple patch

---

diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index 6694d0019a68..c80a631f5f96 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -35,11 +35,11 @@ static inline void sema_init(struct semaphore *sem, int val)
 	lockdep_init_map(&sem->lock.dep_map, "semaphore->lock", &__key, 0);
 }
 
-extern void down(struct semaphore *sem);
+extern bool down(struct semaphore *sem);
 extern int __must_check down_interruptible(struct semaphore *sem);
 extern int __must_check down_killable(struct semaphore *sem);
 extern int __must_check down_trylock(struct semaphore *sem);
 extern int __must_check down_timeout(struct semaphore *sem, long jiffies);
-extern void up(struct semaphore *sem);
+extern bool up(struct semaphore *sem);
 
 #endif /* __LINUX_SEMAPHORE_H */
diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index d9dd94defc0a..daefcbe08147 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -50,16 +50,20 @@ static noinline void __up(struct semaphore *sem);
  * Use of this function is deprecated, please use down_interruptible() or
  * down_killable() instead.
  */
-void down(struct semaphore *sem)
+bool down(struct semaphore *sem)
 {
 	unsigned long flags;
+	bool ret = false;
 
 	raw_spin_lock_irqsave(&sem->lock, flags);
 	if (likely(sem->count > 0))
 		sem->count--;
-	else
+	else {
 		__down(sem);
+		ret = true;
+	}
 	raw_spin_unlock_irqrestore(&sem->lock, flags);
+	return ret;
 }
 EXPORT_SYMBOL(down);
 
@@ -175,16 +179,20 @@ EXPORT_SYMBOL(down_timeout);
  * Release the semaphore.  Unlike mutexes, up() may be called from any
  * context and even by tasks which have never called down().
  */
-void up(struct semaphore *sem)
+bool up(struct semaphore *sem)
 {
 	unsigned long flags;
+	bool ret = false;
 
 	raw_spin_lock_irqsave(&sem->lock, flags);
-	if (likely(list_empty(&sem->wait_list)))
+	if (likely(list_empty(&sem->wait_list))) {
 		sem->count++;
-	else
+	} else {
 		__up(sem);
+		ret = true;
+	}
 	raw_spin_unlock_irqrestore(&sem->lock, flags);
+	return ret;
 }
 EXPORT_SYMBOL(up);
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ca65327a6de8..42ffaaa01859 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -214,12 +214,21 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 /* Number of registered extended console drivers. */
 static int nr_ext_console_drivers;
 
+static void ____down(void)
+{
+	bool sched_out = down(&console_sem);
+	if (sched_out) {
+		pr_err("task %s couldn't lock console_sem and had to schedule out\n", current->comm);
+		dump_stack();
+	}
+}
+
 /*
  * Helper macros to handle lockdep when locking/unlocking console_sem. We use
  * macros instead of functions so that _RET_IP_ contains useful information.
  */
 #define down_console_sem() do { \
-	down(&console_sem);\
+	____down();		\
 	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
 } while (0)
 
@@ -244,15 +253,19 @@ static int __down_trylock_console_sem(unsigned long ip)
 }
 #define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_)
 
-static void __up_console_sem(unsigned long ip)
+static bool __up_console_sem(unsigned long ip)
 {
 	unsigned long flags;
+	bool ret;
 
 	mutex_release(&console_lock_dep_map, 1, ip);
 
 	printk_safe_enter_irqsave(flags);
-	up(&console_sem);
+	ret = up(&console_sem);
+	if (ret)
+		pr_err(">>> %s WOKEUP console_sem() waiter\n", current->comm);
 	printk_safe_exit_irqrestore(flags);
+	return ret;
 }
 #define up_console_sem() __up_console_sem(_RET_IP_)