ext4: fix kernel oops if the journal superblock has a non-zero j_errno

Commit 84061e07 fixed an accounting bug only to introduce the
possibility of a kernel OOPS if the journal has a non-zero j_errno
field indicating that the file system had detected a fs inconsistency.
After the journal replay, if the journal superblock indicates that the
file system has an error, this indication is transfered to the file
system and then ext4_commit_super() is called to write this to the
disk.

But since the percpu counters are now initialized after the journal
replay, the call to ext4_commit_super() will cause a kernel oops since
it needs to use the percpu counters the ext4 superblock structure.

The fix is to skip setting the ext4 free block and free inode fields
if the percpu counter has not been set.

Thanks to Ken Sumrall for reporting and analyzing the root causes of
this bug.

Addresses-Google-Bug: #3054080
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -3964,9 +3964,12 @@ static int ext4_commit_super(struct super_block *sb, int sync)
 	else
 		es->s_kbytes_written =
 			cpu_to_le64(EXT4_SB(sb)->s_kbytes_written);
-	ext4_free_blocks_count_set(es, percpu_counter_sum_positive(
+	if (percpu_counter_initialized(&EXT4_SB(sb)->s_freeblocks_counter))
+		ext4_free_blocks_count_set(es, percpu_counter_sum_positive(
 					&EXT4_SB(sb)->s_freeblocks_counter));
-	es->s_free_inodes_count = cpu_to_le32(percpu_counter_sum_positive(
+	if (percpu_counter_initialized(&EXT4_SB(sb)->s_freeinodes_counter))
+		es->s_free_inodes_count =
+			cpu_to_le32(percpu_counter_sum_positive(
 					&EXT4_SB(sb)->s_freeinodes_counter));
 	sb->s_dirt = 0;
 	BUFFER_TRACE(sbh, "marking dirty");

include/linux/percpu_counter.h

@@ -78,6 +78,11 @@ static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
 	return 1;
 }
 
+static inline int percpu_counter_initialized(struct percpu_counter *fbc)
+{
+	return (fbc->counters != NULL);
+}
+
 #else
 
 struct percpu_counter {
@@ -143,6 +148,11 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
 	return percpu_counter_read(fbc);
 }
 
+static inline int percpu_counter_initialized(struct percpu_counter *fbc)
+{
+	return 1;
+}
+
 #endif	/* CONFIG_SMP */
 
 static inline void percpu_counter_inc(struct percpu_counter *fbc)