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

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Message-ID: <20191119004119.GC208047@google.com>
Date:   Tue, 19 Nov 2019 09:41:19 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Qian Cai <cai@....pw>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (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()?

	-ss