Re: SLOB lockup (was: Re: [tip:core/locking] lockdep: annotate reclaim context (__GFP_NOFS), fix SLOB)

[Nick Piggin]

On Sunday 15 March 2009 17:48:18 Ingo Molnar wrote:

> > Cc: Nick Piggin <npiggin@xxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > LKML-Reference: <20090128135457.350751756@xxxxxxxxx>
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
>
> and with this fixed, and with SLOB now being tested in -tip, the
> new lockdep assert attached below (followed by a real lockup)
> pops up.
>
> Seems like a genuine SLOB bug, probably present upstream as
> well.

Hmmf. debugobjects calls back into the slab allocator from the page
allocator. The following patch would improve SLOB, but I think it
would be a good idea to avoid a dependency in that direction. Can
debugobjects defer this freeing?

---
 mm/slob.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-2.6/mm/slob.c
===================================================================
--- linux-2.6.orig/mm/slob.c
+++ linux-2.6/mm/slob.c
@@ -390,13 +390,14 @@ static void slob_free(void *block, int s
 	spin_lock_irqsave(&slob_lock, flags);
 
 	if (sp->units + units == SLOB_UNITS(PAGE_SIZE)) {
+		spin_lock_irqrestore(&slob_lock, flags);
 		/* Go directly to page allocator. Do not pass slob allocator */
 		if (slob_page_free(sp))
 			clear_slob_page_free(sp);
 		clear_slob_page(sp);
 		free_slob_page(sp);
 		free_page((unsigned long)b);
-		goto out;
+		return;
 	}
 
 	if (!slob_page_free(sp)) {

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