ext4: fix potential NULL dereference while tracing

The allocation_context pointer can be NULL.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -3575,7 +3575,7 @@ ext4_mb_release_inode_pa(struct ext4_buddy *e4b, struct buffer_head *bitmap_bh,
 			trace_ext4_mballoc_discard(ac);
 		}
 
-		trace_ext4_mb_release_inode_pa(ac, pa, grp_blk_start + bit,
+		trace_ext4_mb_release_inode_pa(sb, ac, pa, grp_blk_start + bit,
 					       next - bit);
 		mb_free_blocks(pa->pa_inode, e4b, bit, next - bit);
 		bit = next + 1;
@@ -3606,7 +3606,7 @@ ext4_mb_release_group_pa(struct ext4_buddy *e4b,
 	ext4_group_t group;
 	ext4_grpblk_t bit;
 
-	trace_ext4_mb_release_group_pa(ac, pa);
+	trace_ext4_mb_release_group_pa(sb, ac, pa);
 	BUG_ON(pa->pa_deleted == 0);
 	ext4_get_group_no_and_offset(sb, pa->pa_pstart, &group, &bit);
 	BUG_ON(group != e4b->bd_group && pa->pa_len != 0);

include/trace/events/ext4.h

@@ -395,11 +395,12 @@ DEFINE_EVENT(ext4__mb_new_pa, ext4_mb_new_group_pa,
 );
 
 TRACE_EVENT(ext4_mb_release_inode_pa,
-	TP_PROTO(struct ext4_allocation_context *ac,
+	TP_PROTO(struct super_block *sb,
+		 struct ext4_allocation_context *ac,
 		 struct ext4_prealloc_space *pa,
 		 unsigned long long block, unsigned int count),
 
-	TP_ARGS(ac, pa, block, count),
+	TP_ARGS(sb, ac, pa, block, count),
 
 	TP_STRUCT__entry(
 		__field(	dev_t,	dev			)
@@ -410,8 +411,9 @@ TRACE_EVENT(ext4_mb_release_inode_pa,
 	),
 
 	TP_fast_assign(
-		__entry->dev		= ac->ac_sb->s_dev;
-		__entry->ino		= ac->ac_inode->i_ino;
+		__entry->dev		= sb->s_dev;
+		__entry->ino		= (ac && ac->ac_inode) ? 
+						ac->ac_inode->i_ino : 0;
 		__entry->block		= block;
 		__entry->count		= count;
 	),
@@ -422,10 +424,11 @@ TRACE_EVENT(ext4_mb_release_inode_pa,
 );
 
 TRACE_EVENT(ext4_mb_release_group_pa,
-	TP_PROTO(struct ext4_allocation_context *ac,
+	TP_PROTO(struct super_block *sb,
+		 struct ext4_allocation_context *ac,
 		 struct ext4_prealloc_space *pa),
 
-	TP_ARGS(ac, pa),
+	TP_ARGS(sb, ac, pa),
 
 	TP_STRUCT__entry(
 		__field(	dev_t,	dev			)
@@ -436,8 +439,9 @@ TRACE_EVENT(ext4_mb_release_group_pa,
 	),
 
 	TP_fast_assign(
-		__entry->dev		= ac->ac_sb->s_dev;
-		__entry->ino		= ac->ac_inode->i_ino;
+		__entry->dev		= sb->s_dev;
+		__entry->ino		= (ac && ac->ac_inode) ?
+						ac->ac_inode->i_ino : 0;
 		__entry->pa_pstart	= pa->pa_pstart;
 		__entry->pa_len		= pa->pa_len;
 	),