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Date: Wed, 6 Nov 2013 15:30:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH] mm: create a separate slab for page->ptl allocation
Message-ID: <20131106143037.GO10651@twins.programming.kicks-ass.net>
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On Wed, Nov 06, 2013 at 03:21:55PM +0200, Kirill A. Shutemov wrote:
> On Wed, Nov 06, 2013 at 10:31:31AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 06, 2013 at 01:13:11AM +0200, Kirill A. Shutemov wrote:
> > > I would like to get rid of __ptlock_alloc()/__ptlock_free() too, but I
> > > don't see a way within C: we need to know sizeof(spinlock_t) on
> > > preprocessor stage.
> > > 
> > > We can have a hack on kbuild level: write small helper program to find out
> > > sizeof(spinlock_t) before start building and turn it into define.
> > > But it's overkill from my POV. And cross-compilation will be a fun.
> > 
> > Ah, I just remembered, we have such a thing!
> 
> Great!
> 
> > @@ -1354,7 +1356,7 @@ static inline bool ptlock_init(struct page *page)
> >  	 * slab code uses page->slab_cache and page->first_page (for tail
> >  	 * pages), which share storage with page->ptl.
> >  	 */
> > -	VM_BUG_ON(page->ptl);
> > +	VM_BUG_ON(*(unsigned long *)&page->ptl);
> 
> Huh? Why not direct cast to unsigned long?
> 
> VM_BUG_ON((unsigned long)page->ptl);

I tried, GCC didn't dig that. I think because spinlock_t is a composite
type and you cannot cast that to a primitive type.