[PATCH] slab: don't BUG on duplicated cache

slab presently goes BUG if someone tries to register an already-registered
cache.

But this can happen if the user accidentally loads a module which is already
statically linked into the kernel.  Nuking the kernel is rather a harsh
reaction.

Change it into a warning, and just fail the kmem_cache_alloc() attempt.  If
the module is well-behaved, the modprobe will fail and all is well.

Notes:

- Swaps the ranking of cache_chain_sem and lock_cpu_hotplug().  Doesn't seem
  important.
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -1502,6 +1502,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 {
 	size_t left_over, slab_size, ralign;
 	kmem_cache_t *cachep = NULL;
+	struct list_head *p;
 
 	/*
 	 * Sanity checks... these are all serious usage bugs.
@@ -1516,6 +1517,35 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 			BUG();
 		}
 
+	down(&cache_chain_sem);
+
+	list_for_each(p, &cache_chain) {
+		kmem_cache_t *pc = list_entry(p, kmem_cache_t, next);
+		mm_segment_t old_fs = get_fs();
+		char tmp;
+		int res;
+
+		/*
+		 * This happens when the module gets unloaded and doesn't
+		 * destroy its slab cache and no-one else reuses the vmalloc
+		 * area of the module.  Print a warning.
+		 */
+		set_fs(KERNEL_DS);
+		res = __get_user(tmp, pc->name);
+		set_fs(old_fs);
+		if (res) {
+			printk("SLAB: cache with size %d has lost its name\n",
+					pc->objsize);
+			continue;
+		}
+
+		if (!strcmp(pc->name,name)) {
+			printk("kmem_cache_create: duplicate cache %s\n", name);
+			dump_stack();
+			goto oops;
+		}
+	}
+
 #if DEBUG
 	WARN_ON(strchr(name, ' '));	/* It confuses parsers */
 	if ((flags & SLAB_DEBUG_INITIAL) && !ctor) {
@@ -1592,7 +1622,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 	/* Get cache's description obj. */
 	cachep = (kmem_cache_t *) kmem_cache_alloc(&cache_cache, SLAB_KERNEL);
 	if (!cachep)
-		goto opps;
+		goto oops;
 	memset(cachep, 0, sizeof(kmem_cache_t));
 
 #if DEBUG
@@ -1686,7 +1716,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 		printk("kmem_cache_create: couldn't create cache %s.\n", name);
 		kmem_cache_free(&cache_cache, cachep);
 		cachep = NULL;
-		goto opps;
+		goto oops;
 	}
 	slab_size = ALIGN(cachep->num*sizeof(kmem_bufctl_t)
 				+ sizeof(struct slab), align);
@@ -1781,43 +1811,14 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 		cachep->limit = BOOT_CPUCACHE_ENTRIES;
 	} 
 
-	/* Need the semaphore to access the chain. */
-	down(&cache_chain_sem);
-	{
-		struct list_head *p;
-		mm_segment_t old_fs;
-
-		old_fs = get_fs();
-		set_fs(KERNEL_DS);
-		list_for_each(p, &cache_chain) {
-			kmem_cache_t *pc = list_entry(p, kmem_cache_t, next);
-			char tmp;
-			/* This happens when the module gets unloaded and doesn't
-			   destroy its slab cache and noone else reuses the vmalloc
-			   area of the module. Print a warning. */
-			if (__get_user(tmp,pc->name)) { 
-				printk("SLAB: cache with size %d has lost its name\n", 
-					pc->objsize); 
-				continue; 
-			} 	
-			if (!strcmp(pc->name,name)) { 
-				printk("kmem_cache_create: duplicate cache %s\n",name); 
-				up(&cache_chain_sem); 
-				unlock_cpu_hotplug();
-				BUG(); 
-			}	
-		}
-		set_fs(old_fs);
-	}
-
 	/* cache setup completed, link it into the list */
 	list_add(&cachep->next, &cache_chain);
-	up(&cache_chain_sem);
 	unlock_cpu_hotplug();
-opps:
+oops:
 	if (!cachep && (flags & SLAB_PANIC))
 		panic("kmem_cache_create(): failed to create slab `%s'\n",
 			name);
+	up(&cache_chain_sem);
 	return cachep;
 }
 EXPORT_SYMBOL(kmem_cache_create);