From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>, Jan Kara <jack@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Jan Kara <jack@suse.com>, Tejun Heo <tj@kernel.org>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Byungchul Park <byungchul.park@lge.com>,
vlevenetz@mm-sol.com,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async
Date: Fri, 19 Aug 2016 11:54:55 +0200 [thread overview]
Message-ID: <20160819095455.GR13300@pathway.suse.cz> (raw)
In-Reply-To: <20160819063236.GA584@swordfish>
On Fri 2016-08-19 15:32:36, Sergey Senozhatsky wrote:
> On (08/18/16 12:56), Petr Mladek wrote:
> > The advantage of the printk_func trick is that it is transparent.
> > You do not need to modify any existing functions used by WARN()/BUG()
> > macros.
>
> good point.
>
> so something like below, perhaps. I'm less sure about
> deferred BUG()/BUG_ON():
>
> #define DEFERRED_BUG() do { \
> printk_deferred_enter(); \
> BUG(); \
> printk_deferred_exit(); \
> } while (0) \
>
> #define DEFERRED_BUG_ON(condition) do { \
> printk_deferred_enter(); \
> BUG_ON(condition); \
> printk_deferred_exit(); \
> } while (0)
>
> depending on .config BUG() may never return back -- passing control
> to do_exit(), so printk_deferred_exit() won't be executed. thus we
> probably need to have a per-cpu variable that would indicate that
> we are in deferred_bug. hm... but do we really need deferred BUG()
> in the first place?
Good question. I am not aware of any BUG_ON() that would be called from
wake_up_process() but it is hard to check everything.
A conservative approach would be to force synchronous printk from
BUG_ON().
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
>
> ---
>
> include/asm-generic/bug.h | 50 +++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/printk.h | 6 ++++++
> kernel/printk/internal.h | 18 +----------------
> kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++++++++-----
> 4 files changed, 94 insertions(+), 22 deletions(-)
>
> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index 6f96247..d72ee1e 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -60,6 +60,10 @@ struct bug_entry {
> * significant issues that need prompt attention if they should ever
> * appear at runtime. Use the versions with printk format strings
> * to provide better diagnostics.
> + *
> + * DEFERRED_WARN macros call printk_deferred() to print the messages
> + * and are meant to be used from the contexts where direct printk()
> + * can deadlock the system.
> */
> #ifndef __WARN_TAINT
> extern __printf(3, 4)
> @@ -145,6 +149,52 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
> unlikely(__ret_warn_once); \
> })
>
> +#define DEFERRED_WARN_ON(condition) ({ \
> + int __ret_warn_on = !!(condition); \
> + if (unlikely(__ret_warn_on)) { \
> + printk_deferred_enter(); \
> + __WARN(); \
> + printk_deferred_exit(); \
> + } \
> + unlikely(__ret_warn_on); \
> +})
This looks reasonable to me.
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 0d3e026..6e260a0 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
>
> +void printk_deferred_enter(void)
> +{
> + preempt_disable();
> + this_cpu_write(printk_func, vprintk_deferred);
> +}
> +EXPORT_SYMBOL(printk_deferred_enter);
> +
> +void printk_deferred_exit(void)
> +{
> + this_cpu_write(printk_func, vprintk_default);
> + preempt_enable();
> +}
> +EXPORT_SYMBOL(printk_deferred_exit);
> +
This is racy with printk_nmi_enter() and printk_nmi_exit().
It need to work both ways. It must keep printk_deferred()
even when DEFERRED_WARN_ON() is called in nmi context.
Also it must keep printk_deferred() when the DEFERRED_WARN_ON()
is interrupted by an nmi.
It think that best solution would be to allow nesting.
What about replacing "printk_func" per-CPU variable
with a per-CPU atomic counter. Then we could just
check the counter in vprintk_emit() to see if it is
deferred or not.
The approach with printk_func() was more generic. We thought
that it might be used even for a transparent early_printk().
But it still might be solved even with the per-CPU atomic
counter. We could fallback to the early_printk when a
global flag is set or so.
Best Regards,
Petr
next prev parent reply other threads:[~2016-08-19 9:55 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-04 16:57 [PATCH v10 0/2] printk: Make printk() completely async Sergey Senozhatsky
2016-04-04 16:57 ` [PATCH v10 1/2] " Sergey Senozhatsky
2016-04-04 22:51 ` Andrew Morton
2016-04-05 5:17 ` Sergey Senozhatsky
2016-04-05 7:39 ` Sergey Senozhatsky
2016-04-06 0:19 ` Sergey Senozhatsky
2016-04-06 8:27 ` Jan Kara
2016-04-07 9:48 ` Sergey Senozhatsky
2016-04-07 12:08 ` Sergey Senozhatsky
2016-04-07 13:15 ` Jan Kara
2016-08-10 21:17 ` Viresh Kumar
2016-08-12 9:44 ` Petr Mladek
2016-08-15 14:26 ` Vladislav Levenetz
2016-08-16 9:04 ` Petr Mladek
2016-08-18 2:27 ` Sergey Senozhatsky
2016-08-18 9:33 ` Petr Mladek
2016-08-18 9:51 ` Sergey Senozhatsky
2016-08-18 10:56 ` Petr Mladek
2016-08-19 6:32 ` Sergey Senozhatsky
2016-08-19 9:54 ` Petr Mladek [this message]
2016-08-19 19:00 ` Jan Kara
2016-08-20 5:24 ` Sergey Senozhatsky
2016-08-22 4:15 ` Sergey Senozhatsky
2016-08-23 12:19 ` Petr Mladek
2016-08-24 1:33 ` Sergey Senozhatsky
2016-08-25 21:10 ` Petr Mladek
2016-08-26 1:56 ` Sergey Senozhatsky
2016-08-26 8:20 ` Sergey Senozhatsky
2016-08-30 9:29 ` Petr Mladek
2016-08-31 2:31 ` Sergey Senozhatsky
2016-08-31 9:38 ` Petr Mladek
2016-08-31 12:52 ` Sergey Senozhatsky
2016-09-01 8:58 ` Petr Mladek
2016-09-02 7:58 ` Sergey Senozhatsky
2016-09-02 15:15 ` Petr Mladek
2016-09-06 7:16 ` Sergey Senozhatsky
2016-08-23 13:03 ` Petr Mladek
2016-08-23 13:48 ` Petr Mladek
2016-04-04 16:57 ` [PATCH v10 2/2] printk: Make wake_up_klogd_work_func() async Sergey Senozhatsky
2016-08-23 3:32 [PATCH v10 1/2] printk: Make printk() completely async Andreas Mohr
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