Re: [PATCH printk v1 07/10] console: add write_atomic interface

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Vitor Massaru Iha <vitor@massaru.org>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Changbin Du <changbin.du@intel.com>
Subject: Re: [PATCH printk v1 07/10] console: add write_atomic interface
Date: Tue, 3 Aug 2021 17:02:13 +0300	[thread overview]
Message-ID: <YQlMZT1Isc1zS9A9@smile.fi.intel.com> (raw)
In-Reply-To: <20210803131301.5588-8-john.ogness@linutronix.de>

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