linux-kernel - Re: [patch] slab: Fix a cpu hotplug race condition while tuning slab cpu caches

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Date:	Mon, 16 Oct 2006 12:26:15 -0700
From:	Ravikiran G Thirumalai <kiran@...lex86.org>
To:	Andrew Morton <akpm@...l.org>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Lameter <clameter@...r.sgi.com>,
	Alok Kataria <alok.kataria@...softinc.com>,
	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	"Benzi Galili (Benzi@...leMP.com)" <benzi@...lemp.com>
Subject: Re: [patch] slab: Fix a cpu hotplug race condition while tuning slab cpu caches

On Mon, Oct 16, 2006 at 11:15:11AM -0700, Andrew Morton wrote:
> On Mon, 16 Oct 2006 01:54:39 -0700
> Ravikiran G Thirumalai <kiran@...lex86.org> wrote:
> 
> The problem is obvious: we have some data (the array caches) and we have a
> data structure which is used to look up that data (cpu_online_map).  But
> we're releasing the lock while these two things are in an inconsistent
> state.
> 
> So you could have fixed this by taking cache_chain_mutex in CPU_UP_PREPARE
> and releasing it in CPU_ONLINE and CPU_UP_CANCELED.

Hmm, yes. I suppose so. Maybe we can do away with other uses of
lock_cpu_hotplug() in slab.c as well then!  Will give it a shot. Slab
locking might look uglier than what it already is though no?

> 
> >  	list_for_each_entry(cachep, &cache_chain, next) {
> > @@ -4087,6 +4088,7 @@ ssize_t slabinfo_write(struct file *file
> >  		}
> >  	}
> >  	mutex_unlock(&cache_chain_mutex);
> > +	unlock_cpu_hotplug();
> >  	if (res >= 0)
> >  		res = count;
> >  	return res;
> 
> Given that this lock_cpu_hotplug() happens at a high level I guess it'll
> avoid the usual lock_cpu_hotplug() horrors and we can live with it.  I
> assume lockdep was enabled when you were testing this?

Not when I tested it.  I just retested with lockdep on and things seemed 
fine on a SMP.

Thanks,
Kiran
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