* cache_io() could insert a second page at the same position in the vm_cache

since it unlocked the cache while waiting on a busy page. Now, we're filling
  the pending request before unlocking the cache.
* Fixed the deadlock I mentioned in the last commit: if a page fault happens
  at the same time we're trying to read/write from/to a page, we no longer
  fight for the BFS inode lock, but eventually doing the job twice if needed.
  Will need to go over the "write modified" functions to make sure they are
  behaving as well.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@17091 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/system/kernel/vm/vm.cpp

@@ -2622,6 +2622,12 @@ vm_soft_fault(addr_t originalAddress, bool isWrite, bool isUser)
 			mutex_unlock(&cache_ref->lock);
 
 			page = vm_page_allocate_page(PAGE_STATE_FREE);
+
+			dummy_page.queue_next = page;
+			dummy_page.busy_reading = true;
+				// we mark that page busy reading, so that the file cache can ignore
+				// us in case it works on the very same page
+
 			addressSpace->translation_map.ops->get_physical_page(page->physical_page_number * B_PAGE_SIZE, (addr_t *)&vec.iov_base, PHYSICAL_PAGE_CAN_WAIT);
 			// ToDo: handle errors here
 			err = cache_ref->cache->store->ops->read(cache_ref->cache->store, cacheOffset, &vec, 1, &bytesRead);
@@ -2633,7 +2639,16 @@ vm_soft_fault(addr_t originalAddress, bool isWrite, bool isUser)
 				vm_cache_remove_page(cache_ref, &dummy_page);
 				dummy_page.state = PAGE_STATE_INACTIVE;
 			}
-			vm_cache_insert_page(cache_ref, page, cacheOffset);
+
+			// We insert the queue_next here, because someone else could have
+			// replaced our page
+			vm_cache_insert_page(cache_ref, dummy_page.queue_next, cacheOffset);
+
+			if (dummy_page.queue_next != page) {
+				// Indeed, the page got replaced by someone else - we can safely
+				// throw our page away now
+				vm_page_set_state(page, PAGE_STATE_FREE);
+			}
 			mutex_unlock(&cache_ref->lock);
 			break;
 		}