make vfree() safe to call from interrupt contexts

A bunch of RCU callbacks want to be able to do vfree() and end up with
rather kludgy schemes.  Just let vfree() do the right thing - put the
victim on llist and schedule actual __vunmap() via schedule_work(), so
that it runs from non-interrupt context.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/vmalloc.c

@@ -27,10 +27,30 @@
 #include <linux/pfn.h>
 #include <linux/kmemleak.h>
 #include <linux/atomic.h>
+#include <linux/llist.h>
 #include <asm/uaccess.h>
 #include <asm/tlbflush.h>
 #include <asm/shmparam.h>
 
+struct vfree_deferred {
+	struct llist_head list;
+	struct work_struct wq;
+};
+static DEFINE_PER_CPU(struct vfree_deferred, vfree_deferred);
+
+static void __vunmap(const void *, int);
+
+static void free_work(struct work_struct *w)
+{
+	struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
+	struct llist_node *llnode = llist_del_all(&p->list);
+	while (llnode) {
+		void *p = llnode;
+		llnode = llist_next(llnode);
+		__vunmap(p, 1);
+	}
+}
+
 /*** Page table manipulation functions ***/
 
 static void vunmap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end)
@@ -1184,10 +1204,14 @@ void __init vmalloc_init(void)
 
 	for_each_possible_cpu(i) {
 		struct vmap_block_queue *vbq;
+		struct vfree_deferred *p;
 
 		vbq = &per_cpu(vmap_block_queue, i);
 		spin_lock_init(&vbq->lock);
 		INIT_LIST_HEAD(&vbq->free);
+		p = &per_cpu(vfree_deferred, i);
+		init_llist_head(&p->list);
+		INIT_WORK(&p->wq, free_work);
 	}
 
 	/* Import existing vmlist entries. */
@@ -1520,14 +1544,24 @@ static void __vunmap(const void *addr, int deallocate_pages)
  *	obtained from vmalloc(), vmalloc_32() or __vmalloc(). If @addr is
  *	NULL, no operation is performed.
  *
- *	Must not be called in interrupt context.
+ *	Must not be called in NMI context (strictly speaking, only if we don't
+ *	have CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG, but making the calling
+ *	conventions for vfree() arch-depenedent would be a really bad idea)
+ *	
  */
 void vfree(const void *addr)
 {
-	BUG_ON(in_interrupt());
+	BUG_ON(in_nmi());
 
 	kmemleak_free(addr);
 
+	if (!addr)
+		return;
+	if (unlikely(in_interrupt())) {
+		struct vfree_deferred *p = &__get_cpu_var(vfree_deferred);
+		llist_add((struct llist_node *)addr, &p->list);
+		schedule_work(&p->wq);
+	} else
 		__vunmap(addr, 1);
 }
 EXPORT_SYMBOL(vfree);
@@ -1545,6 +1579,7 @@ void vunmap(const void *addr)
 {
 	BUG_ON(in_interrupt());
 	might_sleep();
+	if (addr)
 		__vunmap(addr, 0);
 }
 EXPORT_SYMBOL(vunmap);