linux-kernel - Re: consoles: was: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog

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Date:   Mon, 7 Dec 2020 10:50:02 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: consoles: was: [PATCH next v2 3/3] printk: remove logbuf_lock,
 add syslog_lock

On Sat 2020-12-05 13:39:53, Sergey Senozhatsky wrote:
> On (20/12/04 17:19), Petr Mladek wrote:
> [..]
> > > --- a/kernel/printk/printk.c
> > > +++ b/kernel/printk/printk.c
> > > @@ -2432,7 +2490,6 @@ void console_unlock(void)
> > >  		size_t len;
> > >  
> > >  		printk_safe_enter_irqsave(flags);
> > 
> > Why do we actually need to use the printk_safe context here?
> > There is not longer a risk of deadlock caused by logbuf_lock.
> > All other recursions should be prevented by console_trylock()
> > in printk().
> 
> All semaphore functions, including down_trylock(), acquire
> semaphore spin_lock;

This has a very easy solution. The patch adds a code that counts
recursion level. We just must not call console_trylock() when
being inside a recursive printk.

printk_safe() has two functions:

  1. It allows to store messages a lockless way. This is obsoleted
     by the lockless ringbuffer.

  2. It prevents calling consoles. We could move this check
     into vprintk_store(). We already have there similar check
     for printk_deferred().


> and then some call into the scheduler
> (or other kernel core functions) under semaphore's spin_lock.
> For instance
> 
> 	up()
> 	 raw_spin_lock_irqsave(&sem->lock)
> 	   __up()
> 	     wake_up_process()
> 	       try_to_wake_up()

This problem is partly solved by printk_deferred(). In each
case, printk_safe() does not help here.

I still do _not_ see a reason to keep printk_safe()!

Best Regards,
Petr