linux-kernel - Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes

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Date:   Tue, 28 Nov 2017 10:42:29 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, kernel-team@....com
Subject: Re: [PATCH v4] printk: Add console owner and waiter logic to load
 balance console writes

On Fri, Nov 24, 2017 at 04:58:16PM +0100, Petr Mladek wrote:
> @@ -1797,13 +1797,6 @@ asmlinkage int vprintk_emit(int facility, int level,
>  				spin_release(&console_owner_dep_map, 1, _THIS_IP_);
>  				printk_safe_exit_irqrestore(flags);
>  
> -				/*
> -				 * The owner passed the console lock to us.
> -				 * Since we did not spin on console lock, annotate
> -				 * this as a trylock. Otherwise lockdep will
> -				 * complain.
> -				 */
> -				mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);

Hello Petr,

IMHO, it would get unbalanced if you only remove this mutex_acquire().

>  				console_unlock();
>  				printk_safe_enter_irqsave(flags);
>  			}
> @@ -2334,10 +2327,10 @@ void console_unlock(void)
>  		/* The waiter is now free to continue */
>  		spin_release(&console_owner_dep_map, 1, _THIS_IP_);
>  		/*
> -		 * Hand off console_lock to waiter. The waiter will perform
> -		 * the up(). After this, the waiter is the console_lock owner.
> +		 * Hand off console_lock to waiter. After this, the waiter
> +		 * is the console_lock owner.
>  		 */
> -		mutex_release(&console_lock_dep_map, 1, _THIS_IP_);

IMHO, this release() should be moved to somewhere properly.

> +		lock_commit_crosslock((struct lockdep_map *)&console_lock_dep_map);
>  		printk_safe_exit_irqrestore(flags);
>  		/* Note, if waiter is set, logbuf_lock is not held */
>  		return;

However, now that cross-release was introduces, lockdep can be applied
to semaphore operations. Actually, I have a plan to do that. I think it
would be better to make semaphore tracked with lockdep and remove all
these manual acquire() and release() here. What do you think about it?

Thanks,
Byungchul