ext4: updated locking documentation for journal_t

Some members of transaction_t are allowed to be read without any lock
being held if consistency doesn't matter.  Based on LockDoc's
findings, we extended the locking documentation of those members.
Each one of them is marked with a short comment: "no lock for quick
racy checks".

Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/ad82c7a9-a624-4ed5-5ada-a6410c44c0b3@tu-dortmund.de
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/jbd2.h

@@ -768,7 +768,8 @@ enum passtype {PASS_SCAN, PASS_REVOKE, PASS_REPLAY};
 struct journal_s
 {
 	/**
-	 * @j_flags: General journaling state flags [j_state_lock]
+	 * @j_flags: General journaling state flags [j_state_lock,
+	 * no lock for quick racy checks]
 	 */
 	unsigned long		j_flags;
 
@@ -808,7 +809,8 @@ struct journal_s
 	/**
 	 * @j_barrier_count:
 	 *
-	 * Number of processes waiting to create a barrier lock [j_state_lock]
+	 * Number of processes waiting to create a barrier lock [j_state_lock,
+	 * no lock for quick racy checks]
 	 */
 	int			j_barrier_count;
 
@@ -821,7 +823,8 @@ struct journal_s
 	 * @j_running_transaction:
 	 *
 	 * Transactions: The current running transaction...
-	 * [j_state_lock] [caller holding open handle]
+	 * [j_state_lock, no lock for quick racy checks] [caller holding
+	 * open handle]
 	 */
 	transaction_t		*j_running_transaction;
 
@@ -1033,15 +1036,15 @@ struct journal_s
 	 * @j_commit_sequence:
 	 *
 	 * Sequence number of the most recently committed transaction
-	 * [j_state_lock].
+	 * [j_state_lock, no lock for quick racy checks]
 	 */
 	tid_t			j_commit_sequence;
 
 	/**
 	 * @j_commit_request:
 	 *
 	 * Sequence number of the most recent transaction wanting commit
-	 * [j_state_lock]
+	 * [j_state_lock, no lock for quick racy checks]
 	 */
 	tid_t			j_commit_request;