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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Byungchul Park <byungchul.park@lge.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	Cong Wang <xiyou.wangcong@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jan Kara <jack@suse.cz>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	rostedt@rostedt.homelinux.com, kernel-team@lge.com
Subject: Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes
Date: Tue, 12 Dec 2017 14:39:21 +0900	[thread overview]
Message-ID: <20171212053921.GA1392@jagdpanzerIV> (raw)
In-Reply-To: <20171208140022.uln4t5e5drrhnvvt@pathway.suse.cz>

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.

WARNING: multiple messages have this Message-ID (diff)
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Byungchul Park <byungchul.park@lge.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	Cong Wang <xiyou.wangcong@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jan Kara <jack@suse.cz>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	rostedt@home.goodmis.org, kernel-team@lge.com
Subject: Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes
Date: Tue, 12 Dec 2017 14:39:21 +0900	[thread overview]
Message-ID: <20171212053921.GA1392@jagdpanzerIV> (raw)
In-Reply-To: <20171208140022.uln4t5e5drrhnvvt@pathway.suse.cz>

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.

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  reply	other threads:[~2017-12-12  5:39 UTC|newest]

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-08 15:27 [PATCH v4] printk: Add console owner and waiter logic to load balance console writes Steven Rostedt
2017-11-08 15:27 ` Steven Rostedt
2017-11-09 10:12 ` Michal Hocko
2017-11-09 10:12   ` Michal Hocko
2017-11-09 10:22   ` [PATCH v4] printk: Add console owner and waiter logic to loadbalance " Tetsuo Handa
2017-11-09 10:22     ` Tetsuo Handa
2017-11-09 10:26     ` Michal Hocko
2017-11-09 10:26       ` Michal Hocko
2017-11-09 11:03       ` Tetsuo Handa
2017-11-09 11:03         ` Tetsuo Handa
2017-11-09 11:31         ` Michal Hocko
2017-11-09 11:31           ` Michal Hocko
2017-11-09 12:07           ` Tetsuo Handa
2017-11-09 12:07             ` Tetsuo Handa
2017-11-24 15:54 ` [PATCH v4] printk: Add console owner and waiter logic to load balance " Petr Mladek
2017-11-24 15:54   ` Petr Mladek
2017-11-24 15:58   ` Petr Mladek
2017-11-24 15:58     ` Petr Mladek
2017-11-27  8:53     ` Byungchul Park
2017-11-27  8:53       ` Byungchul Park
2017-11-28  1:42     ` Byungchul Park
2017-11-28  1:42       ` Byungchul Park
2017-12-08 14:00       ` Petr Mladek
2017-12-08 14:00         ` Petr Mladek
2017-12-12  5:39         ` Sergey Senozhatsky [this message]
2017-12-12  5:39           ` Sergey Senozhatsky
2017-12-12 19:27           ` Steven Rostedt
2017-12-12 19:27             ` Steven Rostedt
2017-12-13  1:50             ` Sergey Senozhatsky
2017-12-13  1:50               ` Sergey Senozhatsky
2017-12-14 14:34             ` Petr Mladek
2017-12-14 14:34               ` Petr Mladek
2017-12-14 13:51     ` Petr Mladek
2017-12-14 13:51       ` Petr Mladek
2017-11-27  8:48 ` Byungchul Park
2017-11-27  8:48   ` Byungchul Park
2017-11-28  6:23   ` Sergey Senozhatsky
2017-11-28  6:23     ` Sergey Senozhatsky
2017-12-22 10:31 ` Petr Mladek
2017-12-22 10:31   ` Petr Mladek
2017-12-22 12:44   ` Steven Rostedt
2017-12-22 12:44     ` Steven Rostedt
2018-01-10 12:50     ` Petr Mladek
2018-01-10 12:50       ` Petr Mladek
  -- strict thread matches above, loose matches on Subject: below --
2017-11-08 15:13 Steven Rostedt
2017-11-08 15:03 Steven Rostedt
2017-11-08 15:10 ` Steven Rostedt

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