linux-kernel - Re: [PATCH] net/skbuff: silence warnings under memory pressure

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Date:   Tue, 19 Nov 2019 10:41:34 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Qian Cai <cai@....pw>, Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure

On Tue 2019-11-19 09:41:19, Sergey Senozhatsky wrote:
> On (19/11/18 16:27), Petr Mladek wrote:
> > > > @@ -2027,8 +2027,11 @@ asmlinkage int vprintk_emit(int facility, int level,
> > > >  	pending_output = (curr_log_seq != log_next_seq);
> > > >  	logbuf_unlock_irqrestore(flags);
> > > >  
> > > > +	if (!pending_output)
> > > > +		return printed_len;
> > > > +
> > > >  	/* If called from the scheduler, we can not call up(). */
> > > > -	if (!in_sched && pending_output) {
> > > > +	if (!in_sched) {
> > > >  		/*
> > > >  		 * Disable preemption to avoid being preempted while holding
> > > >  		 * console_sem which would prevent anyone from printing to
> > > > @@ -2043,10 +2046,11 @@ asmlinkage int vprintk_emit(int facility, int level,
> > > >  		if (console_trylock_spinning())
> > > >  			console_unlock();
> > > >  		preempt_enable();
> > > > -	}
> > > >  
> > > > -	if (pending_output)
> > > > +		wake_up_interruptible(&log_wait);
> > 
> > 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.

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.

Best Regards,
Petr