jbd2: calculate statistics without holding j_state_lock and j_list_lock

The two hottest locks, and thus the biggest scalability bottlenecks,
in the jbd2 layer, are the j_list_lock and j_state_lock.  This has
inspired some people to do some truly unnatural things[1].

[1] https://www.usenix.org/system/files/conference/fast14/fast14-paper_kang.pdf

We don't need to be holding both j_state_lock and j_list_lock while
calculating the journal statistics, so move those calculations to the
very end of jbd2_journal_commit_transaction.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/commit.c

@@ -1083,24 +1083,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 		atomic_read(&commit_transaction->t_handle_count);
 	trace_jbd2_run_stats(journal->j_fs_dev->bd_dev,
 			     commit_transaction->t_tid, &stats.run);
-
-	/*
-	 * Calculate overall stats
-	 */
-	spin_lock(&journal->j_history_lock);
-	journal->j_stats.ts_tid++;
-	if (commit_transaction->t_requested)
-		journal->j_stats.ts_requested++;
-	journal->j_stats.run.rs_wait += stats.run.rs_wait;
-	journal->j_stats.run.rs_request_delay += stats.run.rs_request_delay;
-	journal->j_stats.run.rs_running += stats.run.rs_running;
-	journal->j_stats.run.rs_locked += stats.run.rs_locked;
-	journal->j_stats.run.rs_flushing += stats.run.rs_flushing;
-	journal->j_stats.run.rs_logging += stats.run.rs_logging;
-	journal->j_stats.run.rs_handle_count += stats.run.rs_handle_count;
-	journal->j_stats.run.rs_blocks += stats.run.rs_blocks;
-	journal->j_stats.run.rs_blocks_logged += stats.run.rs_blocks_logged;
-	spin_unlock(&journal->j_history_lock);
+	stats.ts_requested = (commit_transaction->t_requested) ? 1 : 0;
 
 	commit_transaction->t_state = T_COMMIT_CALLBACK;
 	J_ASSERT(commit_transaction == journal->j_committing_transaction);
@@ -1157,4 +1140,21 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	spin_unlock(&journal->j_list_lock);
 	write_unlock(&journal->j_state_lock);
 	wake_up(&journal->j_wait_done_commit);
+
+	/*
+	 * Calculate overall stats
+	 */
+	spin_lock(&journal->j_history_lock);
+	journal->j_stats.ts_tid++;
+	journal->j_stats.ts_requested += stats.ts_requested;
+	journal->j_stats.run.rs_wait += stats.run.rs_wait;
+	journal->j_stats.run.rs_request_delay += stats.run.rs_request_delay;
+	journal->j_stats.run.rs_running += stats.run.rs_running;
+	journal->j_stats.run.rs_locked += stats.run.rs_locked;
+	journal->j_stats.run.rs_flushing += stats.run.rs_flushing;
+	journal->j_stats.run.rs_logging += stats.run.rs_logging;
+	journal->j_stats.run.rs_handle_count += stats.run.rs_handle_count;
+	journal->j_stats.run.rs_blocks += stats.run.rs_blocks;
+	journal->j_stats.run.rs_blocks_logged += stats.run.rs_blocks_logged;
+	spin_unlock(&journal->j_history_lock);
 }