linux-kernel - Re: linux.git: printk() problem

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Date:   Tue, 25 Oct 2016 11:24:36 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Joe Perches <joe@...ches.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: linux.git: printk() problem

On (10/25/16 10:55), Sergey Senozhatsky wrote:
> I think cont_flush() should grab the logbuf_lock lock, because
> it does log_store() and touches the cont.len. so something like
> this perhaps
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index c7f490f..47f887c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1608,13 +1608,20 @@ static struct cont {
>  
>  static bool cont_flush(void)
>  {
> +       unsigned long flags;
> +       bool flushed = false;
> +
> +       raw_spin_lock_irqsave(&logbuf_lock, flags);
>         if (!cont.len)
> -               return false;
> +               goto out;
>  
>         log_store(cont.facility, cont.level, cont.flags, cont.ts_nsec,
>                   NULL, 0, cont.buf, cont.len);
>         cont.len = 0;
> -       return true;
> +       flushed = true;
> +out:
> +       raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> +       return flushed;
>  }


... clearly, wasn't tested at all! sorry about that.
what I meant was cont_flush() from deferred_cont_flush()->flush_timer().


diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c7f490f..bd7841c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1619,7 +1619,14 @@ static bool cont_flush(void)
 
 static void flush_timer(unsigned long data)
 {
-	if (cont_flush())
+	unsigned long flags;
+	bool flushed;
+
+	raw_spin_lock_irqsave(&logbuf_lock, flags);
+	flushed = cont_flush();
+	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+
+	if (flushed)
 		wake_up_klogd();
 }

 	-ss