linux-kernel - Re: [PATCH printk v1 07/10] console: add write

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Date:   Tue, 3 Aug 2021 17:02:13 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Vitor Massaru Iha <vitor@...saru.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Changbin Du <changbin.du@...el.com>
Subject: Re: [PATCH printk v1 07/10] console: add write_atomic interface

On Tue, Aug 03, 2021 at 03:18:58PM +0206, John Ogness wrote:
> Add a write_atomic() callback to the console. This is an optional
> function for console drivers. The function must be atomic (including
> NMI safe) for writing to the console.
> 
> Console drivers implementing write_atomic() must select the new
> HAVE_ATOMIC_CONSOLE Kconfig option.
> 
> Console drivers must still implement the write() callback. The
> write_atomic() callback will only be used in special situations,
> such as when the kernel panics.
> 
> Creating an NMI safe write_atomic() that must synchronize with
> write() requires a careful implementation of the console driver. To
> aid with the implementation, a set of console_atomic_*() functions
> are provided:
> 
>     void console_atomic_lock(unsigned long flags);
>     void console_atomic_unlock(unsigned long flags);
> 
> These functions synchronize using the printk cpulock and disable
> hardware interrupts.
> 
> In order to increase effectiveness, the printk cpulock functions are
> also made more aggressive and now keep interrupts disabled while
> spinning.
> 
> CPUs holding the printk cpulock must not spin on any other lock.
> Therefore can_use_console() will now return false if the printk
> cpulock is held in order to avoid calling into console driver code,
> while typically contain spinlocks.
> 
> Likewise, console_trylock_spinning() will fail rather than attempt
> to acquire the console_sem (which includes a spinlock in its

...

>  #include <linux/atomic.h>
>  #include <linux/types.h>
> +#include <linux/printk.h>

Ordered?

...

> +			if (!(con->flags & CON_ENABLED))	\
> +				continue;			\

What about

#define console_is_enabled(con)		(!!(con->flags & CON_ENABLED))

or inliner equivalent

static inline bool console_is_enabled(struct console *con)
{
	return !!(con->flags & CON_ENABLED);
}

?

-- 
With Best Regards,
Andy Shevchenko