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

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Message-ID: <20190906195135.GA69785@tigerII.localdomain>
Date:   Sat, 7 Sep 2019 04:51:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Qian Cai <cai@....pw>,
        davem@...emloft.net, Eric Dumazet <eric.dumazet@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure

On (09/06/19 16:55), Petr Mladek wrote:
> > I think we can queue significantly much less irq_work-s from printk().
> > 
> > Petr, Steven, what do you think?
> > 
> > Something like this. Call wake_up_interruptible(), switch to
> > wake_up_klogd() only when called from sched code.
> 
> Replacing irq_work_queue() with wake_up_interruptible() looks
> dangerous to me.
> 
> As a result, all "normal" printk() calls from the scheduler
> code will deadlock. There is almost always a userspace
> logger registered.

I don't see why all printk()-s should deadlock.

A "normal" printk() call will deadlock only when scheduler calls
"normal" printk() under rq or pi locks. But this is illegal anyway,
because console_sem up() calls wake_up_process() - the same function
wake_up_interruptible() calls. IOW "normal" printk() calls from
scheduler end up in scheduler, via console_sem->sched chain. We
already execute wake_up_process()->try_to_wake_up() in printk(),
even when a non-LOGLEVEL_SCHED printk() comes from scheduler.

What am I missing something?

	-ss