jbd2: don't use state lock during commit path

Variables journal->j_fc_off, journal->j_fc_wbuf are accessed during
commit path. Since today we allow only one process to perform a fast
commit, there is no need take state lock before accessing these
variables. This patch removes these locks and adds comments to
describe this.
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-9-harshadshirwadkar@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/journal.c

@@ -865,7 +865,6 @@ int jbd2_fc_get_buf(journal_t *journal, struct buffer_head **bh_out)
 	int fc_off;
 
 	*bh_out = NULL;
-	write_lock(&journal->j_state_lock);
 
 	if (journal->j_fc_off + journal->j_fc_first < journal->j_fc_last) {
 		fc_off = journal->j_fc_off;
@@ -874,7 +873,6 @@ int jbd2_fc_get_buf(journal_t *journal, struct buffer_head **bh_out)
 	} else {
 		ret = -EINVAL;
 	}
-	write_unlock(&journal->j_state_lock);
 
 	if (ret)
 		return ret;
@@ -909,9 +907,7 @@ int jbd2_fc_wait_bufs(journal_t *journal, int num_blks)
 	struct buffer_head *bh;
 	int i, j_fc_off;
 
-	read_lock(&journal->j_state_lock);
 	j_fc_off = journal->j_fc_off;
-	read_unlock(&journal->j_state_lock);
 
 	/*
 	 * Wait in reverse order to minimize chances of us being woken up before
@@ -939,9 +935,7 @@ int jbd2_fc_release_bufs(journal_t *journal)
 	struct buffer_head *bh;
 	int i, j_fc_off;
 
-	read_lock(&journal->j_state_lock);
 	j_fc_off = journal->j_fc_off;
-	read_unlock(&journal->j_state_lock);
 
 	/*
 	 * Wait in reverse order to minimize chances of us being woken up before

include/linux/jbd2.h

@@ -945,8 +945,9 @@ struct journal_s
 	/**
 	 * @j_fc_off:
 	 *
-	 * Number of fast commit blocks currently allocated.
-	 * [j_state_lock].
+	 * Number of fast commit blocks currently allocated. Accessed only
+	 * during fast commit. Currently only process can do fast commit, so
+	 * this field is not protected by any lock.
 	 */
 	unsigned long		j_fc_off;
 
@@ -1109,8 +1110,9 @@ struct journal_s
 	struct buffer_head	**j_wbuf;
 
 	/**
-	 * @j_fc_wbuf: Array of fast commit bhs for
-	 * jbd2_journal_commit_transaction.
+	 * @j_fc_wbuf: Array of fast commit bhs for fast commit. Accessed only
+	 * during a fast commit. Currently only process can do fast commit, so
+	 * this field is not protected by any lock.
 	 */
 	struct buffer_head	**j_fc_wbuf;