@@ -30,6 +30,7 @@ static const struct fscache_state *fscache_look_up_object(struct fscache_object
static const struct fscache_state *fscache_object_available(struct fscache_object *, int);
static const struct fscache_state *fscache_parent_ready(struct fscache_object *, int);
static const struct fscache_state *fscache_update_object(struct fscache_object *, int);
+static const struct fscache_state *fscache_object_dead(struct fscache_object *, int);
#define __STATE_NAME(n) fscache_osm_##n
#define STATE(n) (&__STATE_NAME(n))
@@ -91,7 +92,7 @@ static WORK_STATE(LOOKUP_FAILURE, "LCFL", fscache_lookup_failure);
static WORK_STATE(KILL_OBJECT, "KILL", fscache_kill_object);
static WORK_STATE(KILL_DEPENDENTS, "KDEP", fscache_kill_dependents);
static WORK_STATE(DROP_OBJECT, "DROP", fscache_drop_object);
-static WORK_STATE(OBJECT_DEAD, "DEAD", (void*)2UL);
+static WORK_STATE(OBJECT_DEAD, "DEAD", fscache_object_dead);
static WAIT_STATE(WAIT_FOR_INIT, "?INI",
TRANSIT_TO(INIT_OBJECT, 1 << FSCACHE_OBJECT_EV_NEW_CHILD));
@@ -229,6 +230,10 @@ static void fscache_object_sm_dispatcher(struct fscache_object *object)
event = -1;
if (new_state == NO_TRANSIT) {
_debug("{OBJ%x} %s notrans", object->debug_id, state->name);
+ if (unlikely(state == STATE(OBJECT_DEAD))) {
+ _leave(" [dead]");
+ return;
+ }
fscache_enqueue_object(object);
event_mask = object->oob_event_mask;
goto unmask_events;
@@ -239,7 +244,7 @@ static void fscache_object_sm_dispatcher(struct fscache_object *object)
object->state = state = new_state;
if (state->work) {
- if (unlikely(state->work == ((void *)2UL))) {
+ if (unlikely(state == STATE(OBJECT_DEAD))) {
_leave(" [dead]");
return;
}
@@ -1083,3 +1088,20 @@ void fscache_object_mark_killed(struct fscache_object *object,
}
}
EXPORT_SYMBOL(fscache_object_mark_killed);
+
+/*
+ * The object is dead. We can get here if an object gets queued by an event
+ * that would lead to its death (such as EV_KILL) when the dispatcher is
+ * already running (and so can be requeued) but hasn't yet cleared the event
+ * mask.
+ */
+static const struct fscache_state *fscache_object_dead(struct fscache_object *object,
+ int event)
+{
+ if (!test_and_set_bit(FSCACHE_OBJECT_RUN_AFTER_DEAD,
+ &object->flags))
+ return NO_TRANSIT;
+
+ WARN(true, "FS-Cache object redispatched after death");
+ return NO_TRANSIT;
+}
@@ -360,6 +360,7 @@ struct fscache_object {
#define FSCACHE_OBJECT_IS_AVAILABLE 5 /* T if object has become active */
#define FSCACHE_OBJECT_RETIRED 6 /* T if object was retired on relinquishment */
#define FSCACHE_OBJECT_KILLED_BY_CACHE 7 /* T if object was killed by the cache */
+#define FSCACHE_OBJECT_RUN_AFTER_DEAD 8 /* T if object has been dispatched after death */
struct list_head cache_link; /* link in cache->object_list */
struct hlist_node cookie_link; /* link in cookie->backing_objects */
Under some circumstances, an fscache object can become queued such that it fscache_object_work_func() can be called once the object is in the OBJECT_DEAD state. This results in the kernel oopsing when it tries to invoke the handler for the state (which is hard coded to 0x2). The way this comes about is something like the following: (1) The object dispatcher is processing a work state for an object. This is done in workqueue context. (2) An out-of-band event comes in that isn't masked, causing the object to be queued, say EV_KILL. (3) The object dispatcher finishes processing the current work state on that object and then sees there's another event to process, so, without returning to the workqueue core, it processes that event too. It then follows the chain of events that initiates until we reach OBJECT_DEAD without going through a wait state (such as WAIT_FOR_CLEARANCE). At this point, object->events may be 0, object->event_mask will be 0 and oob_event_mask will be 0. (4) The object dispatcher returns to the workqueue processor, and in due course, this sees that the object's work item is still queued and invokes it again. (5) The current state is a work state (OBJECT_DEAD), so the dispatcher jumps to it - resulting in an OOPS. When I'm seeing this, the work state in (1) appears to have been either LOOK_UP_OBJECT or CREATE_OBJECT (object->oob_table is fscache_osm_lookup_oob). The window for (2) is very small: (A) object->event_mask is cleared whilst the event dispatch process is underway - though there's no memory barrier to force this to the top of the function. The window, therefore is from the time the object was selected by the workqueue processor and made requeueable to the time the mask was cleared. (B) fscache_raise_event() will only queue the object if it manages to set the event bit and the corresponding event_mask bit was set. The enqueuement is then deferred slightly whilst we get a ref on the object and get the per-CPU variable for workqueue congestion. This slight deferral slightly increases the probability by allowing extra time for the workqueue to make the item requeueable. Handle this by giving the dead state a processor function and checking the for the dead state address rather than seeing if the processor function is address 0x2. The dead state processor function can then set a flag to indicate that it's occurred and give a warning if it occurs more than once per object. If this race occurs, an oops similar to the following is seen (note the RIP value): BUG: unable to handle kernel NULL pointer dereference at 0000000000000002 IP: [<0000000000000002>] 0x1 PGD 0 Oops: 0010 [#1] SMP Modules linked in: ... CPU: 17 PID: 16077 Comm: kworker/u48:9 Not tainted 3.10.0-327.18.2.el7.x86_64 #1 Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 12/27/2015 Workqueue: fscache_object fscache_object_work_func [fscache] task: ffff880302b63980 ti: ffff880717544000 task.ti: ffff880717544000 RIP: 0010:[<0000000000000002>] [<0000000000000002>] 0x1 RSP: 0018:ffff880717547df8 EFLAGS: 00010202 RAX: ffffffffa0368640 RBX: ffff880edf7a4480 RCX: dead000000200200 RDX: 0000000000000002 RSI: 00000000ffffffff RDI: ffff880edf7a4480 RBP: ffff880717547e18 R08: 0000000000000000 R09: dfc40a25cb3a4510 R10: dfc40a25cb3a4510 R11: 0000000000000400 R12: 0000000000000000 R13: ffff880edf7a4510 R14: ffff8817f6153400 R15: 0000000000000600 FS: 0000000000000000(0000) GS:ffff88181f420000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000002 CR3: 000000000194a000 CR4: 00000000001407e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Stack: ffffffffa0363695 ffff880edf7a4510 ffff88093f16f900 ffff8817faa4ec00 ffff880717547e60 ffffffff8109d5db 00000000faa4ec18 0000000000000000 ffff8817faa4ec18 ffff88093f16f930 ffff880302b63980 ffff88093f16f900 Call Trace: [<ffffffffa0363695>] ? fscache_object_work_func+0xa5/0x200 [fscache] [<ffffffff8109d5db>] process_one_work+0x17b/0x470 [<ffffffff8109e4ac>] worker_thread+0x21c/0x400 [<ffffffff8109e290>] ? rescuer_thread+0x400/0x400 [<ffffffff810a5acf>] kthread+0xcf/0xe0 [<ffffffff810a5a00>] ? kthread_create_on_node+0x140/0x140 [<ffffffff816460d8>] ret_from_fork+0x58/0x90 [<ffffffff810a5a00>] ? kthread_create_on_node+0x140/0x140 Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Jeremy McNicoll <jeremymc@redhat.com> Tested-by: Frank Sorenson <sorenson@redhat.com> Tested-by: Benjamin Coddington <bcodding@redhat.com> Reviewed-by: Benjamin Coddington <bcodding@redhat.com> --- fs/fscache/object.c | 26 ++++++++++++++++++++++++-- include/linux/fscache-cache.h | 1 + 2 files changed, 25 insertions(+), 2 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html