cachefiles: unbind cachefiles gracefully in on-demand mode

Add a refcount to avoid the deadlock in on-demand read mode. The
on-demand read mode will pin the corresponding cachefiles object for
each anonymous fd. The cachefiles object is unpinned when the anonymous
fd gets closed. When the user daemon exits and the fd of
"/dev/cachefiles" device node gets closed, it will wait for all
cahcefiles objects getting withdrawn. Then if there's any anonymous fd
getting closed after the fd of the device node, the user daemon will
hang forever, waiting for all objects getting withdrawn.

To fix this, add a refcount indicating if there's any object pinned by
anonymous fds. The cachefiles cache gets unbound and withdrawn when the
refcount is decreased to 0. It won't change the behaviour of the
original mode, in which case the cachefiles cache gets unbound and
withdrawn as long as the fd of the device node gets closed.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220509074028.74954-4-jefflexu@linux.alibaba.com
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/fs/cachefiles/daemon.c b/fs/cachefiles/daemon.c
index d5417da7f792..5b1d0642c749 100644
--- a/fs/cachefiles/daemon.c
+++ b/fs/cachefiles/daemon.c
@@ -111,6 +111,7 @@ static int cachefiles_daemon_open(struct inode *inode, struct file *file)
 	INIT_LIST_HEAD(&cache->volumes);
 	INIT_LIST_HEAD(&cache->object_list);
 	spin_lock_init(&cache->object_list_lock);
+	refcount_set(&cache->unbind_pincount, 1);
 	xa_init_flags(&cache->reqs, XA_FLAGS_ALLOC);
 	xa_init_flags(&cache->ondemand_ids, XA_FLAGS_ALLOC1);
 
@@ -164,6 +165,20 @@ static void cachefiles_flush_reqs(struct cachefiles_cache *cache)
 	xa_destroy(&cache->ondemand_ids);
 }
 
+void cachefiles_put_unbind_pincount(struct cachefiles_cache *cache)
+{
+	if (refcount_dec_and_test(&cache->unbind_pincount)) {
+		cachefiles_daemon_unbind(cache);
+		cachefiles_open = 0;
+		kfree(cache);
+	}
+}
+
+void cachefiles_get_unbind_pincount(struct cachefiles_cache *cache)
+{
+	refcount_inc(&cache->unbind_pincount);
+}
+
 /*
  * Release a cache.
  */
@@ -179,14 +194,12 @@ static int cachefiles_daemon_release(struct inode *inode, struct file *file)
 
 	if (cachefiles_in_ondemand_mode(cache))
 		cachefiles_flush_reqs(cache);
-	cachefiles_daemon_unbind(cache);
 
 	/* clean up the control file interface */
 	cache->cachefilesd = NULL;
 	file->private_data = NULL;
-	cachefiles_open = 0;
 
-	kfree(cache);
+	cachefiles_put_unbind_pincount(cache);
 
 	_leave("");
 	return 0;