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Date:   Thu, 2 Nov 2017 17:53:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "yuwang.yuwang" <yuwang.yuwang@...baba-inc.com>
Subject: Re: [PATCH] mm: don't warn about allocations which stall for too long

On (10/31/17 15:32), Steven Rostedt wrote:
[..]
> (new globals)
> static DEFINE_SPIN_LOCK(console_owner_lock);
> static struct task_struct console_owner;
> static bool waiter;
> 
> console_unlock() {
> 
> [ Assumes this part can not preempt ]
>
> 	spin_lock(console_owner_lock);
> 	console_owner = current;
> 	spin_unlock(console_owner_lock);

 + disables IRQs?

> 	for each message
> 		write message out to console
> 
> 		if (READ_ONCE(waiter))
> 			break;
> 
> 	spin_lock(console_owner_lock);
> 	console_owner = NULL;
> 	spin_unlock(console_owner_lock);
> 
> [ preemption possible ]

otherwise

     printk()
      if (console_trylock())
        console_unlock()
         preempt_disable()
          spin_lock(console_owner_lock);
          console_owner = current;
          spin_unlock(console_owner_lock);
          .......
          spin_lock(console_owner_lock);
IRQ
    printk()
     console_trylock() // fails so we go to busy-loop part
      spin_lock(console_owner_lock);       << deadlock


even if we would replace spin_lock(console_owner_lock) with IRQ
spin_lock, we still would need to protect against IRQs on the very
same CPU. right? IOW, we need to store smp_processor_id() of a CPU
currently doing console_unlock() and check it in vprintk_emit()?
and we need to protect the entire console_unlock() function. not
just the printing loop, otherwise the IRQ CPU will spin forever
waiting for itself to up() the console_sem.

this somehow reminds me of "static unsigned int logbuf_cpu", which
we used to have in vprintk_emit() and were happy to remove it...


the whole "console_unlock() is non-preemptible" can bite, I'm
afraid. it's not always printk()->console_unlock(), sometimes
it's console_lock()->console_unlock() that has to flush the
logbuf.

CPU0					CPU1  ~  CPU99
console_lock();
					printk(); ... printk();
console_unlock()
 preempt_disable();
  for (;;)
    call_console_drivers();
    <<lockup>>


this pattern is not so unusual. _especially_ in the existing scheme
of things.

not to mention the problem of "the last printk()", which will take
over and do the flush.

CPU0					CPU1  ~  CPU99
console_lock();
					printk(); ... printk();
console_unlock();
					    IRQ on CPU2
					     printk()
					      // take over console_sem
					      console_unlock()

and so on.
seems that there will be lots of if-s.

	-ss

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