* The method vm_cache_remove_consumer() and the page daemon used to get

a reference to a by them not yet referenced cache was not correct.
  They only incremented the reference count, but a vnode cache reference
  includes also a vnode reference. In case of the page daemon this would
  cause vnode references to be lost (causing bug #1465).
* The page daemon used an unsafe method to access a yet unreferenced
  page cache. There was nothing that prevented the cache from being
  deleted while the page daemon tried to get a reference. The
  vm_page::cache field is now protected by the page cache table
  spinlock, too, which the new function
  vm_cache_acquire_page_cache_ref(), used by the page daemon, also
  acquires while trying to get the reference.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@22208 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/vm_cache.h

@@ -23,6 +23,7 @@ status_t vm_cache_init(struct kernel_args *args);
 vm_cache *vm_cache_create(vm_store *store);
 void vm_cache_acquire_ref(vm_cache *cache);
 void vm_cache_release_ref(vm_cache *cache);
+vm_cache *vm_cache_acquire_page_cache_ref(vm_page *page);
 vm_page *vm_cache_lookup_page(vm_cache *cache, off_t page);
 void vm_cache_insert_page(vm_cache *cache, vm_page *page, off_t offset);
 void vm_cache_remove_page(vm_cache *cache, vm_page *page);

src/system/kernel/vm/vm_cache.cpp

@@ -11,6 +11,9 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#include <util/khash.h>
+#include <util/AutoLock.h>
+
 #include <arch/cpu.h>
 #include <condition_variable.h>
 #include <debug.h>
@@ -18,7 +21,6 @@
 #include <kernel.h>
 #include <lock.h>
 #include <smp.h>
-#include <util/khash.h>
 #include <vm.h>
 #include <vm_page.h>
 #include <vm_priv.h>
@@ -80,6 +82,28 @@ page_hash_func(void *_p, const void *_key, uint32 range)
 }
 
 
+/*!	Acquires a pseudo reference to a cache yet unreferenced by the caller. The
+	caller must make sure, that the cache is not deleted, e.g. by holding the
+	cache's source cache lock or by holding the page cache table lock while the
+	cache is still referred to by a page. To get a real reference, the caller
+	must subsequently call vm_cache_acquire_ref() and decrement the cache's ref
+	count manually afterwards.
+	Returns \c true, if the pseudo reference could be acquired.
+*/
+static inline bool
+acquire_unreferenced_cache_pseudo_ref(vm_cache* cache)
+{
+	while (true) {
+		int32 count = cache->ref_count;
+		if (count == 0)
+			return false;
+
+		if (atomic_test_and_set(&cache->ref_count, count + 1, count) == count)
+			return true;
+	}
+}
+
+
 status_t
 vm_cache_init(kernel_args *args)
 {
@@ -275,6 +299,29 @@ vm_cache_release_ref(vm_cache *cache)
 }
 
 
+vm_cache*
+vm_cache_acquire_page_cache_ref(vm_page* page)
+{
+	InterruptsSpinLocker locker(sPageCacheTableLock);
+
+	vm_cache* cache = page->cache;
+	if (cache == NULL)
+		return NULL;
+
+	// get a pseudo reference
+	if (!acquire_unreferenced_cache_pseudo_ref(cache))
+		return NULL;
+
+	locker.Unlock();
+
+	// turn it into a real reference
+	vm_cache_acquire_ref(cache);
+	atomic_add(&cache->ref_count, -1);
+
+	return cache;
+}
+
+
 vm_page *
 vm_cache_lookup_page(vm_cache *cache, off_t offset)
 {
@@ -326,12 +373,13 @@ vm_cache_insert_page(vm_cache *cache, vm_page *page, off_t offset)
 	cache->page_list = page;
 	cache->page_count++;
 
-	page->cache = cache;
 	page->usage_count = 2;
 
 	state = disable_interrupts();
 	acquire_spinlock(&sPageCacheTableLock);
 
+	page->cache = cache;
+
 #if KDEBUG
 	struct page_lookup_key key;
 	key.offset = (uint32)(offset >> PAGE_SHIFT);
@@ -373,6 +421,7 @@ vm_cache_remove_page(vm_cache *cache, vm_page *page)
 	acquire_spinlock(&sPageCacheTableLock);
 
 	hash_remove(sPageCacheTable, page);
+	page->cache = NULL;
 
 	release_spinlock(&sPageCacheTableLock);
 	restore_interrupts(state);
@@ -388,7 +437,6 @@ vm_cache_remove_page(vm_cache *cache, vm_page *page)
 			page->cache_next->cache_prev = page->cache_prev;
 	}
 	cache->page_count--;
-	page->cache = NULL;
 }
 
 
@@ -499,27 +547,23 @@ vm_cache_remove_consumer(vm_cache *cache, vm_cache *consumer)
 		&& cache->consumers.link.next == cache->consumers.link.prev) {
 		// The cache is not really needed anymore - it can be merged with its only
 		// consumer left.
-		bool merge = false;
 
 		consumer = (vm_cache *)list_get_first_item(&cache->consumers);
 
-		// Our cache doesn't have a ref to its consumer (only the other way around), 
-		// so we cannot just acquire it here; it might be deleted right now
-		while (true) {
-			int32 count = consumer->ref_count;
-			if (count == 0)
-				break;
-
-			if (atomic_test_and_set(&consumer->ref_count, count + 1, count) == count) {
-				// We managed to grab a reference to the consumerRef.
-				// Since this doesn't guarantee that we get the cache we wanted
-				// to, we need to check if this cache is really the last
-				// consumer of the cache we want to merge it with.
-				merge = true;
-				break;
-			}
+		bool merge = acquire_unreferenced_cache_pseudo_ref(consumer);
+		if (merge) {
+			// We managed to increment the reference count, but that's not a
+			// full reference. We get a real one now and decrement the ref count
+			// again.
+			vm_cache_acquire_ref(consumer);
+			atomic_add(&consumer->ref_count, -1);
 		}
 
+		// In case we managed to grab a reference to the consumerRef,
+		// this doesn't guarantee that we get the cache we wanted
+		// to, so we need to check if this cache is really the last
+		// consumer of the cache we want to merge it with.
+
 		ConditionVariable<vm_cache> busyCondition;
 
 		if (merge) {

src/system/kernel/vm/vm_daemons.cpp

@@ -70,21 +70,8 @@ PageCacheLocker::Lock(vm_page* page)
 	if (_IgnorePage(page))
 		return false;
 
-	vm_cache* cache = page->cache;
-
-	// Grab a reference to this cache - the page does not own a reference
-	// to its cache, so we can't just acquire it the easy way
-	while (true) {
-		int32 count = cache->ref_count;
-		if (count == 0) {
-			cache = NULL;
-			break;
-		}
-
-		if (atomic_test_and_set(&cache->ref_count, count + 1, count) == count)
-			break;
-	}
-
+	// Grab a reference to this cache.
+	vm_cache* cache = vm_cache_acquire_page_cache_ref(page);
 	if (cache == NULL)
 		return false;