linux-kernel - Re: [PATCH next v1 2/3] printk: remove safe buffers

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Date:   Sun, 21 Mar 2021 14:26:10 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Alistair Popple <alistair@...ple.id.au>,
        Jordan Niethe <jniethe5@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Cédric Le Goater <clg@...d.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, Yue Hu <huyue2@...ong.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Rafael Aquini <aquini@...hat.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        "Guilherme G. Piccoli" <gpiccoli@...onical.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org, kexec@...ts.infradead.org
Subject: Re: [PATCH next v1 2/3] printk: remove safe buffers

On (21/03/17 00:33), John Ogness wrote:
[..]
>  void printk_nmi_direct_enter(void)
>  {
> @@ -324,27 +44,8 @@ void printk_nmi_direct_exit(void)
>  	this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
>  }
>  
> -#else
> -
> -static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
> -{
> -	return 0;
> -}
> -
>  #endif /* CONFIG_PRINTK_NMI */
>  
> -/*
> - * Lock-less printk(), to avoid deadlocks should the printk() recurse
> - * into itself. It uses a per-CPU buffer to store the message, just like
> - * NMI.
> - */
> -static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
> -{
> -	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
> -
> -	return printk_safe_log_store(s, fmt, args);
> -}
> -
>  /* Can be preempted by NMI. */
>  void __printk_safe_enter(void)
>  {
> @@ -369,7 +70,10 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
>  	 * Use the main logbuf even in NMI. But avoid calling console
>  	 * drivers that might have their own locks.
>  	 */
> -	if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK)) {
> +	if (this_cpu_read(printk_context) &
> +	    (PRINTK_NMI_DIRECT_CONTEXT_MASK |
> +	     PRINTK_NMI_CONTEXT_MASK |
> +	     PRINTK_SAFE_CONTEXT_MASK)) {

Do we need printk_nmi_direct_enter/exit() and PRINTK_NMI_DIRECT_CONTEXT_MASK?
Seems like all printk_safe() paths are now DIRECT - we store messages to the
prb, but don't call console drivers.

	-ss