linux-kernel - Re: [PATCH] lockdep: Add a document describing crossrelease feature

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Date:	Wed, 6 Jul 2016 09:56:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	Boqun Feng <boqun.feng@...il.com>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, walken@...gle.com
Subject: Re: [PATCH] lockdep: Add a document describing crossrelease feature

On Wed, Jul 06, 2016 at 02:33:29PM +0900, Byungchul Park wrote:
> On Wed, Jul 06, 2016 at 11:17:10AM +0900, Byungchul Park wrote:
> > 
> > lock(A)
> > wait_for(B)
> > ~~~~~~~~~~~~~~~~~~~~~~~~ <- serialized by atomic operation
> > 		lock(A)
> > 		unlock(A)
> > 		wake(B)
> > unlock(A)
> 
> By the way, I have a question. Is there anyone who could answer it?
> 
> I want to serialize between two context's lock operations, for example,
> 
> 	context A	context B
> 	--------------	--------------
> 	lock A
> 	lock B		...
> 	lock C
> 	atomic_inc_return
> 	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <- serialization
> 			atomic_read
> 			lock D
> 	...		lock E
> 			lock F
> 
> so that we can see these in the order like A -> B -> C -> D -> E -> F.
> 
> atomic_inc_return() is used after lock C in context A, and atomic_read()
> is used before lock D in context B. And I want to make it serialized when
> the atomic_read() can see the increased value.
> 
> Can I use smp_mb__after_atomic() just after atomic_read() 

No. atomic_set() and atomic_read() are not RmW operations.

> or should I use
> smp_mb()? I think anyway I have to choose one of them for that ordering.

smp_load_acquire(), if that observes the increment it will ensure D
comes after etc..

Also, atomic_read() _could_ be enough, if its part of a control
dependency, because LOCK very much involves a store, so the load->store
order provided by the control dependency will already order things.