linux-kernel - Re: [PATCH v2 2/6] oom: Get rid of sparse warnings

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Message-ID: <20120207062353.GA24687@oksana.dev.rtsoft.ru>
Date:	Tue, 7 Feb 2012 10:23:53 +0400
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	San Mehat <san@...gle.com>, Colin Cross <ccross@...roid.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v2 2/6] oom: Get rid of sparse warnings

On Tue, Feb 07, 2012 at 12:38:58AM -0500, KOSAKI Motohiro wrote:
> >> task struct only have allock_lock, not alloc_loc.
> >
> > Funnily, but sparse does not care. :-) __release(foo) will work as
> > well. Seems like sparse counts locking balance globally.
> >
> > This is now fixed in the patch down below, thanks for catching.
> >
> >> Moreover we don't release
> >> the lock in this code path. Seems odd.
> >
> > Indeed. That's exactly what sparse seeing is as well. We exit
> > without releasing the lock, which is bad (in sparse' eyes). So
> > we lie to sparse, telling it that we do release, so it shut ups.
> 
> Hmmm....
> 
> To be honest, I really dislike any lie annotation. Why? It is very
> fragile and easily
> become broken from unrelated but near line changes. Please consider to
> enhance sparse at first.

I somewhat doubt that it is possible to "enhance it". We keep the
lock held conditionaly, so we need too place the hint in the
code itself. I believe the best we can do would be something like
this:

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4a24354..61d91f2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -14,6 +14,7 @@
 # define __releases(x)	__attribute__((context(x,1,0)))
 # define __acquire(x)	__context__(x,1)
 # define __release(x)	__context__(x,-1)
+# define __ret_with_lock(x)	__context__(x,-1)
 # define __cond_lock(x,c)	((c) ? ({ __acquire(x); 1; }) : 0)
 # define __percpu	__attribute__((noderef, address_space(3)))
 #ifdef CONFIG_SPARSE_RCU_POINTER
@@ -37,6 +38,7 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 # define __releases(x)
 # define __acquire(x) (void)0
 # define __release(x) (void)0
+# define __ret_with_lock(x)
 # define __cond_lock(x,c) (c)
 # define __percpu
 # define __rcu

And then use it instead of __release().

-- 
Anton Vorontsov
Email: cbouatmailru@...il.com
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