Re: [BUG] Lockdep recursive locking in kmem_cache_free
From: Ravikiran G Thirumalai
Date: Sat Jul 29 2006 - 00:22:59 EST
On Fri, Jul 28, 2006 at 02:26:16PM -0700, Christoph Lameter wrote:
> On Fri, 28 Jul 2006, Ravikiran G Thirumalai wrote:
>
> > Why should there be any problem taking the remote l3 lock? If the remote
> > node does not have cpu that does not mean we cannot take a lock from the
> > local node!!!
> >
> > I think current git does not teach lockdep to ignore recursion for
> > array_cache->lock when the array_cache->lock are from different cases. As
> > Arjan pointed out, I can see that l3->list_lock is special cased, but I
> > cannot find where array_cache->lock is taken care of.
>
> Ok.
>
> > Again, if this is indeed a problem (recursion) machine should not boot even,
> > when compiled without lockdep, tglx, can you please verify this?
>
> We seem to be fine on that level.
>
Since false positives due to off slab slab management seem to occur
often of late, how about adding some comments to slab.c?
Thanks,
Kiran
---
Adds some comments to slab.c.
Also, checks if we get a valid slabp_cache for off slab slab-descriptors.
We should always get this. If we don't, then in that case we,
will have to disable off-slab descriptors for this cache and do the
calculations again. This is a rare case, so add a BUG_ON, for now,
just in case.
Signed-off-by: Alok N Kataria <alok.kataria@xxxxxxxxxxxxxx>
Signed-off-by: Ravikiran Thirumalai <kiran@xxxxxxxxxxxx>
Signed-off-by: Shai Fultheim <shai@xxxxxxxxxxxx>
Index: linux-2.6.18-rc2.git/mm/slab.c
===================================================================
--- linux-2.6.18-rc2.git.orig/mm/slab.c 2006-07-28 17:43:04.000000000 -0700
+++ linux-2.6.18-rc2.git/mm/slab.c 2006-07-28 18:13:31.000000000 -0700
@@ -2200,8 +2200,17 @@ kmem_cache_create (const char *name, siz
cachep->gfpflags |= GFP_DMA;
cachep->buffer_size = size;
- if (flags & CFLGS_OFF_SLAB)
+ if (flags & CFLGS_OFF_SLAB) {
cachep->slabp_cache = kmem_find_general_cachep(slab_size, 0u);
+ /*
+ * This is a possibility for one of the malloc_sizes caches.
+ * But since we go off slab only for object size greater than
+ * PAGE_SIZE/8, and malloc_sizes gets created in ascending order,
+ * this should not happen at all.
+ * But leave a BUG_ON for some lucky dude.
+ */
+ BUG_ON(!cachep->slabp_cache);
+ }
cachep->ctor = ctor;
cachep->dtor = dtor;
cachep->name = name;
@@ -2435,7 +2444,17 @@ int kmem_cache_destroy(struct kmem_cache
}
EXPORT_SYMBOL(kmem_cache_destroy);
-/* Get the memory for a slab management obj. */
+/*
+ * Get the memory for a slab management obj.
+ * For a slab cache when the slab descriptor is off-slab, slab descriptors
+ * always come from malloc_sizes caches. The slab descriptor cannot
+ * come from the same cache which is getting created because,
+ * when we are searching for an appropriate cache for these
+ * descriptors in kmem_cache_create, we search through the malloc_sizes array.
+ * If we are creating a malloc_sizes cache here it would not be visible to
+ * kmem_find_general_cachep till the initialization is complete.
+ * Hence we cannot have slabp_cache same as the original cache.
+ */
static struct slab *alloc_slabmgmt(struct kmem_cache *cachep, void *objp,
int colour_off, gfp_t local_flags,
int nodeid)
@@ -3119,6 +3138,12 @@ static void free_block(struct kmem_cache
if (slabp->inuse == 0) {
if (l3->free_objects > l3->free_limit) {
l3->free_objects -= cachep->num;
+ /* No need to drop any previously held
+ * lock here, even if we have a off-slab slab
+ * descriptor it is guaranteed to come from
+ * a different cache, refer to comments before
+ * alloc_slabmgmt.
+ */
slab_destroy(cachep, slabp);
} else {
list_add(&slabp->list, &l3->slabs_free);
-
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