linux-kernel - Re: [PATCH 1/2] printk: Add an option to flush all messages on panic

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Message-ID: <20190402024717.az4bjtmfd7yhyaom@shbuild888>
Date:   Tue, 2 Apr 2019 10:47:17 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Mukesh Ojha <mojha@...eaurora.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 1/2] printk: Add an option to flush all messages on panic

Hi Mukesh,

On Mon, Apr 01, 2019 at 05:20:33PM +0530, Mukesh Ojha wrote:
> 
> On 4/1/2019 4:18 PM, Feng Tang wrote:
> > Current console_flush_on_panic() will only dump the new messages
> > in buffer, and users may need an opportunity to check all the
> > messages on panic which could help debugging, as user may haven't
> > seen the log before panic due to loglevel settings.
> > 
> > Add a flag for console_flush_on_panic() to chose whether to
> > dump all messages.
> > 
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> 
> 
> Looks good to me.

Thanks for the review.

> 
> 
> But, Will it not be good if put it under a config and not change the
> prototype, as it is a debug feature?

My understanding is the console_flush_on_panic() is also a debug
feature too :), and my 2/2 patch will has a bitmask (either controlled
by kernel cmdline or sysctl) to turn on this.

Thanks,
Feng


> 
> Thanks,
> Mukesh
> 
> > ---
> >   arch/powerpc/kernel/traps.c | 2 +-
> >   include/linux/console.h     | 2 +-
> >   kernel/panic.c              | 2 +-
> >   kernel/printk/printk.c      | 9 ++++++++-
> >   4 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> > index 1fd45a8..58d9580 100644
> > --- a/arch/powerpc/kernel/traps.c
> > +++ b/arch/powerpc/kernel/traps.c
> > @@ -179,7 +179,7 @@ extern void panic_flush_kmsg_end(void)
> >   	kmsg_dump(KMSG_DUMP_PANIC);
> >   	bust_spinlocks(0);
> >   	debug_locks_off();
> > -	console_flush_on_panic();
> > +	console_flush_on_panic(false);
> >   }
> >   static unsigned long oops_begin(struct pt_regs *regs)
> > diff --git a/include/linux/console.h b/include/linux/console.h
> > index ec9bdb3..825ecf5 100644
> > --- a/include/linux/console.h
> > +++ b/include/linux/console.h
> > @@ -175,7 +175,7 @@ extern int console_trylock(void);
> >   extern void console_unlock(void);
> >   extern void console_conditional_schedule(void);
> >   extern void console_unblank(void);
> > -extern void console_flush_on_panic(void);
> > +extern void console_flush_on_panic(bool flush_all);
> >   extern struct tty_driver *console_device(int *);
> >   extern void console_stop(struct console *);
> >   extern void console_start(struct console *);
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index 0ae0d73..fb77e01 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -277,7 +277,7 @@ void panic(const char *fmt, ...)
> >   	 * panic() is not being callled from OOPS.
> >   	 */
> >   	debug_locks_off();
> > -	console_flush_on_panic();
> > +	console_flush_on_panic(false);
> >   	panic_print_sys_info();
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 02ca827..8ff099b 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2525,10 +2525,11 @@ void console_unblank(void)
> >   /**
> >    * console_flush_on_panic - flush console content on panic
> > + * @flush_all:	whether to print all messages in buffer
> >    *
> >    * Immediately output all pending messages no matter what.
> >    */
> > -void console_flush_on_panic(void)
> > +void console_flush_on_panic(bool flush_all)
> >   {
> >   	/*
> >   	 * If someone else is holding the console lock, trylock will fail
> > @@ -2539,6 +2540,12 @@ void console_flush_on_panic(void)
> >   	 */
> >   	console_trylock();
> >   	console_may_schedule = 0;
> > +
> > +	/* User may want to see all the printk messages on panic */
> > +	if (flush_all) {
> > +		console_seq = log_first_seq;
> > +		console_idx = log_first_idx;
> > +	}
> >   	console_unlock();
> >   }