mm/shmem: fix potential dead loop in shmem_unuse()

If multi shmem_unuse() for different swap type is called concurrently, a
dead loop could occur as following:

shmem_unuse(typeA)               shmem_unuse(typeB)
 mutex_lock(&shmem_swaplist_mutex)
 list_for_each_entry_safe(info, next, ...)
  ...
  mutex_unlock(&shmem_swaplist_mutex)
  /* info->swapped may drop to 0 */
  shmem_unuse_inode(&info->vfs_inode, type)

                                  mutex_lock(&shmem_swaplist_mutex)
                                  list_for_each_entry(info, next, ...)
                                   if (!info->swapped)
                                    list_del_init(&info->swaplist)

                                  ...
                                  mutex_unlock(&shmem_swaplist_mutex)

  mutex_lock(&shmem_swaplist_mutex)
  /* iterate with offlist entry and encounter a dead loop */
  next = list_next_entry(info, swaplist);
  ...

Restart the iteration if the inode is already off shmem_swaplist list to
fix the issue.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b56a2d8 ("mm: rid swapoff of quadratic complexity")
Signed-off-by: Kemeng Shi <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: kernel test robot <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kemeng Shi

mm/shmem.c

@@ -1505,6 +1505,7 @@ int shmem_unuse(unsigned int type)
 		return 0;
 
 	mutex_lock(&shmem_swaplist_mutex);
+start_over:
 	list_for_each_entry_safe(info, next, &shmem_swaplist, swaplist) {
 		if (!info->swapped) {
 			list_del_init(&info->swaplist);
@@ -1523,13 +1524,15 @@ int shmem_unuse(unsigned int type)
 		cond_resched();
 
 		mutex_lock(&shmem_swaplist_mutex);
-		next = list_next_entry(info, swaplist);
-		if (!info->swapped)
-			list_del_init(&info->swaplist);
 		if (atomic_dec_and_test(&info->stop_eviction))
 			wake_up_var(&info->stop_eviction);
 		if (error)
 			break;
+		if (list_empty(&info->swaplist))
+			goto start_over;
+		next = list_next_entry(info, swaplist);
+		if (!info->swapped)
+			list_del_init(&info->swaplist);
 	}
 	mutex_unlock(&shmem_swaplist_mutex);