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SubjectRe: recursive slqb_lock
On Thu, Feb 26, 2009 at 01:34:50PM +0800, Wu Fengguang wrote:
> Hi Nick,
>
> I got a lockdep warning. It looks like the slqb_lock will be taken
> twice in the call chain:
>
> s_start() => take slqb_lock
> s_show()
> gather_stats() => take slqb_lock again

Hey, thanks for this. The following patch should fix it.

--
Fix the lockdep error reported by Fengguang where down_read slqb_lock
is taken twice, with the possibility for a deadlock.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/slqb.c
===================================================================
--- linux-2.6.orig/mm/slqb.c 2009-03-02 02:05:16.000000000 +1100
+++ linux-2.6/mm/slqb.c 2009-03-02 02:05:45.000000000 +1100
@@ -3079,7 +3079,9 @@ static void __gather_stats(void *arg)
spin_unlock(&gather->lock);
}

-static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
+/* must be called with slqb_lock held */
+static void gather_stats_locked(struct kmem_cache *s,
+ struct stats_gather *stats)
{
#ifdef CONFIG_NUMA
int node;
@@ -3089,8 +3091,6 @@ static void gather_stats(struct kmem_cac
stats->s = s;
spin_lock_init(&stats->lock);

- down_read(&slqb_lock); /* hold off hotplug */
-
on_each_cpu(__gather_stats, stats, 1);

#ifdef CONFIG_NUMA
@@ -3119,10 +3119,15 @@ static void gather_stats(struct kmem_cac
}
#endif

- up_read(&slqb_lock);
-
stats->nr_objects = stats->nr_slabs * s->objects;
}
+
+static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
+{
+ down_read(&slqb_lock); /* hold off hotplug */
+ gather_stats(s, stats);
+ up_read(&slqb_lock);
+}
#endif

/*
@@ -3175,7 +3180,7 @@ static int s_show(struct seq_file *m, vo

s = list_entry(p, struct kmem_cache, list);

- gather_stats(s, &stats);
+ gather_stats_locked(s, &stats);

seq_printf(m, "%-17s %6lu %6lu %6u %4u %4d", s->name, stats.nr_inuse,
stats.nr_objects, s->size, s->objects, (1 << s->order));

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