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@xxxxxxxxxxxx> 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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