From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.com>, Tejun Heo <tj@kernel.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
linux-kernel@vger.kernel.org,
Byungchul Park <byungchul.park@lge.com>, Jan Kara <jack@suse.cz>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async
Date: Fri, 1 Apr 2016 18:36:02 +0900 [thread overview]
Message-ID: <20160401093602.GB503@swordfish> (raw)
In-Reply-To: <20160401085912.GO5522@pathway.suse.cz>
Hello Petr,
On (04/01/16 10:59), Petr Mladek wrote:
[..]
> CPU0 CPU1
>
> printk()
>
> if (printk_kthread)
> # fails and need_flush_console
> # stays false
>
> init_printk_kthread()
> # put printk_thread into
> # run queue
> printk_kthread = ...;
>
> if (!in_panic && printk_kthread)
> wake_up_process(printk_kthread);
>
>
> # printk kthread finally gets
> # scheduled
> printk_kthread_func()
>
> set_current_state(TASK_INTERRUPTIBLE);
> if (!need_flush_console)
> schedule();
>
> => printk_kthread is happily sleeping without calling console.
oohh, that tiny race. well, looks quite harmless, it's unlikely that
we had printk()-s up until late_initcall(init_printk_kthread) and not
a single one ever after. but good find!
so the check
if (printk_kthread)
need_flush_console = 1
can be replaced with
if (!printk_sync)
need_flush_console = 1
or... may be dropped.
> I do not see any code that will modify need_flush_console when
> printk.synchronous is modified at runtime.
printk.synchronous is RO; no runtime modification.
> I know that all this is rather theoretical. My main point is to remove
> unnecessary checks that make the code harder to read and does not bring
> any big advantage.
my point is that those checks are just .loads, which help to avoid
spurious .stores from various CPUs.
CPU1 CPU2 CPU3 ... CPU1024
lock logbuf
need_flush=1
unlock logbuf
lock logbuf
need_flush=1
unlock logbuf
lock logbuf
need_flush=1
unlock logbuf
wakeup kthread
...
lock logbuf
need_flush=1
unlock logbuf
isn't it a bit useless need_flush=1 traffic?
-ss
next prev parent reply other threads:[~2016-04-01 9:36 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-24 15:43 [RFC][PATCH v8 0/2] printk: Make printk() completely async Sergey Senozhatsky
2016-03-24 15:43 ` [RFC][PATCH v8 1/2] " Sergey Senozhatsky
2016-03-29 9:14 ` Jan Kara
2016-04-01 1:14 ` Sergey Senozhatsky
2016-03-31 11:12 ` Petr Mladek
2016-03-31 11:52 ` Jan Kara
2016-04-04 9:41 ` Petr Mladek
2016-04-04 17:01 ` Sergey Senozhatsky
2016-04-01 1:08 ` Sergey Senozhatsky
2016-04-01 8:59 ` Petr Mladek
2016-04-01 9:36 ` Sergey Senozhatsky [this message]
2016-04-01 11:30 ` Petr Mladek
2016-03-24 15:43 ` [RFC][PATCH v8 2/2] printk: Make wake_up_klogd_work_func() async Sergey Senozhatsky
2016-03-31 11:14 ` Petr Mladek
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