[PATCH] xen/grant-table: annotate on-stack work in gnttab_unmap_refs_sync()

[Kavitha Daula <kavi@edera.dev>]

Both callers of gnttab_unmap_refs_sync(), free_persistent_gnts() and
xen_blkbk_unmap_purged_grants() in blkback, keep their struct
gntab_unmap_queue_data on the stack. gnttab_unmap_refs_async() sets up
the delayed work inside it with INIT_DELAYED_WORK(), so with
CONFIG_DEBUG_OBJECTS_WORK and CONFIG_DEBUG_OBJECTS_TIMERS every
persistent grant teardown warns twice, once for the work and once for
its timer:

  [  104.366085] ODEBUG: object ffffc9004020fad8 is on stack ffffc9004020c000, but NOT annotated.
  [  104.371551] WARNING: CPU: 1 PID: 48 at lib/debugobjects.c:672 lookup_object_or_alloc.part.0+0x12b/0x1f0
  Call Trace:
   <TASK>
   __debug_object_init+0xfe/0x1a0
   debug_object_init+0x38/0x50
   __init_work+0x28/0x30
   gnttab_unmap_refs_async+0x18/0xa0
   gnttab_unmap_refs_sync+0x61/0x90
   free_persistent_gnts+0x179/0x280
   xen_blkbk_free_caches+0x16/0x30
   xen_blkif_disconnect+0xfa/0x2d0
   frontend_changed+0x90/0x680
   xenbus_otherend_changed+0x80/0xb0
   frontend_changed+0x14/0x20
   xenwatch_thread+0x96/0x1a0
   kthread+0x11d/0x250
   ret_from_fork+0x29f/0x2d0
   ret_from_fork_asm+0x1a/0x30
   </TASK>

gnttab_unmap_refs_sync() waits for the unmap to complete before it
returns, so the work never outlives the caller's frame. Initialise it
with INIT_DELAYED_WORK_ONSTACK() when the item is on the stack and
destroy it before returning. The function is exported, so an item that
is not on the stack keeps INIT_DELAYED_WORK().

Fixes: b44166cd46e2 ("xen/grant: introduce func gnttab_unmap_refs_sync()")
Signed-off-by: Kavitha Daula <kavi@edera.dev>
---
 drivers/xen/grant-table.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index f75b5cc..5a85908 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -38,6 +38,7 @@
 #include <linux/sched.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
+#include <linux/sched/task_stack.h>
 #include <linux/vmalloc.h>
 #include <linux/uaccess.h>
 #include <linux/io.h>
@@ -1360,12 +1361,21 @@ static void unmap_refs_callback(int result,
 int gnttab_unmap_refs_sync(struct gntab_unmap_queue_data *item)
 {
 	struct unmap_refs_callback_data data;
+	bool on_stack = object_is_on_stack(item);
 
 	init_completion(&data.completion);
 	item->data = &data;
 	item->done = &unmap_refs_callback;
-	gnttab_unmap_refs_async(item);
+	/* blkback keeps @item, and so its retry work, on the stack. */
+	if (on_stack)
+		INIT_DELAYED_WORK_ONSTACK(&item->gnttab_work, gnttab_unmap_work);
+	else
+		INIT_DELAYED_WORK(&item->gnttab_work, gnttab_unmap_work);
+	item->age = 0;
+	__gnttab_unmap_refs_async(item);
 	wait_for_completion(&data.completion);
+	if (on_stack)
+		destroy_delayed_work_on_stack(&item->gnttab_work);
 
 	return data.result;
 }
-- 
2.50.1 (Apple Git-155)