FS-Cache: Simplify cookie retention for fscache_objects, fixing oops

Simplify the way fscache cache objects retain their cookie.  The way I
implemented the cookie storage handling made synchronisation a pain (ie. the
object state machine can't rely on the cookie actually still being there).

Instead of the the object being detached from the cookie and the cookie being
freed in __fscache_relinquish_cookie(), we defer both operations:

 (*) The detachment of the object from the list in the cookie now takes place
     in fscache_drop_object() and is thus governed by the object state machine
     (fscache_detach_from_cookie() has been removed).

 (*) The release of the cookie is now in fscache_object_destroy() - which is
     called by the cache backend just before it frees the object.

This means that the fscache_cookie struct is now available to the cache all the
way through from ->alloc_object() to ->drop_object() and ->put_object() -
meaning that it's no longer necessary to take object->lock to guarantee access.

However, __fscache_relinquish_cookie() doesn't wait for the object to go all
the way through to destruction before letting the netfs proceed.  That would
massively slow down the netfs.  Since __fscache_relinquish_cookie() leaves the
cookie around, in must therefore break all attachments to the netfs - which
includes ->def, ->netfs_data and any outstanding page read/writes.

To handle this, struct fscache_cookie now has an n_active counter:

 (1) This starts off initialised to 1.

 (2) Any time the cache needs to get at the netfs data, it calls
     fscache_use_cookie() to increment it - if it is not zero.  If it was zero,
     then access is not permitted.

 (3) When the cache has finished with the data, it calls fscache_unuse_cookie()
     to decrement it.  This does a wake-up on it if it reaches 0.

 (4) __fscache_relinquish_cookie() decrements n_active and then waits for it to
     reach 0.  The initialisation to 1 in step (1) ensures that we only get
     wake ups when we're trying to get rid of the cookie.

This leaves __fscache_relinquish_cookie() a lot simpler.


***
This fixes a problem in the current code whereby if fscache_invalidate() is
followed sufficiently quickly by fscache_relinquish_cookie() then it is
possible for __fscache_relinquish_cookie() to have detached the cookie from the
object and cleared the pointer before a thread is dispatched to process the
invalidation state in the object state machine.

Since the pending write clearance was deferred to the invalidation state to
make it asynchronous, we need to either wait in relinquishment for the stores
tree to be cleared in the invalidation state or we need to handle the clearance
in relinquishment.

Further, if the relinquishment code does clear the tree, then the invalidation
state need to make the clearance contingent on still having the cookie to hand
(since that's where the tree is rooted) and we have to prevent the cookie from
disappearing for the duration.

This can lead to an oops like the following:

BUG: unable to handle kernel NULL pointer dereference at 000000000000000c
...
RIP: 0010:[<ffffffff8151023e>] _spin_lock+0xe/0x30
...
CR2: 000000000000000c ...
...
Process kslowd002 (...)
....
Call Trace:
 [<ffffffffa01c3278>] fscache_invalidate_writes+0x38/0xd0 [fscache]
 [<ffffffff810096f0>] ? __switch_to+0xd0/0x320
 [<ffffffff8105e759>] ? find_busiest_queue+0x69/0x150
 [<ffffffff8110ddd4>] ? slow_work_enqueue+0x104/0x180
 [<ffffffffa01c1303>] fscache_object_slow_work_execute+0x5e3/0x9d0 [fscache]
 [<ffffffff81096b67>] ? bit_waitqueue+0x17/0xd0
 [<ffffffff8110e233>] slow_work_execute+0x233/0x310
 [<ffffffff8110e515>] slow_work_thread+0x205/0x360
 [<ffffffff81096ca0>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff8110e310>] ? slow_work_thread+0x0/0x360
 [<ffffffff81096936>] kthread+0x96/0xa0
 [<ffffffff8100c0ca>] child_rip+0xa/0x20
 [<ffffffff810968a0>] ? kthread+0x0/0xa0
 [<ffffffff8100c0c0>] ? child_rip+0x0/0x20

The parameter to fscache_invalidate_writes() was object->cookie which is NULL.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-By: Milosz Tanski <milosz@adfin.com>
Acked-by: Jeff Layton <jlayton@redhat.com>

1 files changed, 40 insertions, 53 deletions

diff --git a/fs/fscache/object-list.c b/fs/fscache/object-list.c
index 4a386b080e0..e1959efad64 100644
--- a/fs/fscache/object-list.c
+++ b/fs/fscache/object-list.c
@@ -70,13 +70,10 @@ void fscache_objlist_add(struct fscache_object *obj)
 	write_unlock(&fscache_object_list_lock);
 }
 
-/**
- * fscache_object_destroy - Note that a cache object is about to be destroyed
- * @object: The object to be destroyed
- *
- * Note the imminent destruction and deallocation of a cache object record.
+/*
+ * Remove an object from the object list.
  */
-void fscache_object_destroy(struct fscache_object *obj)
+void fscache_objlist_remove(struct fscache_object *obj)
 {
 	write_lock(&fscache_object_list_lock);
 
@@ -85,7 +82,6 @@ void fscache_object_destroy(struct fscache_object *obj)
 
 	write_unlock(&fscache_object_list_lock);
 }
-EXPORT_SYMBOL(fscache_object_destroy);
 
 /*
  * find the object in the tree on or after the specified index
@@ -166,10 +162,9 @@ static int fscache_objlist_show(struct seq_file *m, void *v)
 {
 	struct fscache_objlist_data *data = m->private;
 	struct fscache_object *obj = v;
+	struct fscache_cookie *cookie;
 	unsigned long config = data->config;
-	uint16_t keylen, auxlen;
 	char _type[3], *type;
-	bool no_cookie;
 	u8 *buf = data->buf, *p;
 
 	if ((unsigned long) v == 1) {
@@ -216,8 +211,9 @@ static int fscache_objlist_show(struct seq_file *m, void *v)
 		}							\
 	} while(0)
 
+	cookie = obj->cookie;
 	if (~config) {
-		FILTER(obj->cookie,
+		FILTER(cookie->def,
 		       COOKIE, NOCOOKIE);
 		FILTER(fscache_object_is_active(obj) ||
 		       obj->n_ops != 0 ||
@@ -250,48 +246,40 @@ static int fscache_objlist_show(struct seq_file *m, void *v)
 		   obj->flags,
 		   work_busy(&obj->work));
 
-	no_cookie = true;
-	keylen = auxlen = 0;
-	if (obj->cookie) {
-		spin_lock(&obj->lock);
-		if (obj->cookie) {
-			switch (obj->cookie->def->type) {
-			case 0:
-				type = "IX";
-				break;
-			case 1:
-				type = "DT";
-				break;
-			default:
-				sprintf(_type, "%02u",
-					obj->cookie->def->type);
-				type = _type;
-				break;
-			}
+	if (fscache_use_cookie(obj)) {
+		uint16_t keylen = 0, auxlen = 0;
 
-			seq_printf(m, "%-16s %s %2lx %16p",
-				   obj->cookie->def->name,
-				   type,
-				   obj->cookie->flags,
-				   obj->cookie->netfs_data);
-
-			if (obj->cookie->def->get_key &&
-			    config & FSCACHE_OBJLIST_CONFIG_KEY)
-				keylen = obj->cookie->def->get_key(
-					obj->cookie->netfs_data,
-					buf, 400);
-
-			if (obj->cookie->def->get_aux &&
-			    config & FSCACHE_OBJLIST_CONFIG_AUX)
-				auxlen = obj->cookie->def->get_aux(
-					obj->cookie->netfs_data,
-					buf + keylen, 512 - keylen);
-
-			no_cookie = false;
+		switch (cookie->def->type) {
+		case 0:
+			type = "IX";
+			break;
+		case 1:
+			type = "DT";
+			break;
+		default:
+			sprintf(_type, "%02u", cookie->def->type);
+			type = _type;
+			break;
 		}
-		spin_unlock(&obj->lock);
 
-		if (!no_cookie && (keylen > 0 || auxlen > 0)) {
+		seq_printf(m, "%-16s %s %2lx %16p",
+			   cookie->def->name,
+			   type,
+			   cookie->flags,
+			   cookie->netfs_data);
+
+		if (cookie->def->get_key &&
+		    config & FSCACHE_OBJLIST_CONFIG_KEY)
+			keylen = cookie->def->get_key(cookie->netfs_data,
+						      buf, 400);
+
+		if (cookie->def->get_aux &&
+		    config & FSCACHE_OBJLIST_CONFIG_AUX)
+			auxlen = cookie->def->get_aux(cookie->netfs_data,
+						      buf + keylen, 512 - keylen);
+		fscache_unuse_cookie(obj);
+
+		if (keylen > 0 || auxlen > 0) {
 			seq_printf(m, " ");
 			for (p = buf; keylen > 0; keylen--)
 				seq_printf(m, "%02x", *p++);
@@ -302,12 +290,11 @@ static int fscache_objlist_show(struct seq_file *m, void *v)
 					seq_printf(m, "%02x", *p++);
 			}
 		}
-	}
 
-	if (no_cookie)
-		seq_printf(m, "<no_cookie>\n");
-	else
 		seq_printf(m, "\n");
+	} else {
+		seq_printf(m, "<no_netfs>\n");
+	}
 	return 0;
 }