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

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Date:   Sat, 5 Dec 2020 13:39:53 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...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 (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; 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()

	-ss