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

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Date:   Tue, 12 Dec 2017 14:39:21 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Byungchul Park <byungchul.park@....com>,
        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

Hello,

On (12/08/17 15:00), Petr Mladek wrote:
[..]
> > 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?
> 
> IMHO, it would be great to add lockdep annotations into semaphore
> operations.

certain types of locks have no guaranteed lock-unlock ordering.
e.g. readers-writer locks, semaphores, etc.

for readers-writer lock we can easily have

CPU0		CPU1		CPU2		CPU3		CPU4
read_lock
		write_lock
		// sleep because
		// of CPU0
								read_lock
read_unlock			read_lock
				read_unlock	read_lock
						read_unlock
								read_unlock
								// wake up CPU1

so for CPU1 the lock was "locked" by CPU0 and "unlocked" by CPU4.

semaphore not necessarily has the mutual-exclusion property, because
its ->count is not required to be set to 1. in printk we use semaphore
with ->count == 1, but that's just an accident.

	-ss


p.s.
frankly, I don't see any "locking issues" in Steven's patch.