ext4: akpm's locking hack to fix locking delays

This is a port of the following patch from Andrew Morton to ext4:

	http://lkml.org/lkml/2008/10/3/22

This fixes a major contention problem in do_get_write_access() when a
buffer is modified in both the current and committing transaction.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: akpm@linux-foundation.org
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 0aea14c..f039c97 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -543,6 +543,7 @@ do {									       \
 #define EXT4_MOUNT_I_VERSION            0x2000000 /* i_version support */
 #define EXT4_MOUNT_DELALLOC		0x8000000 /* Delalloc support */
 #define EXT4_MOUNT_DATA_ERR_ABORT	0x10000000 /* Abort on file data write */
+#define EXT4_MOUNT_AKPM_LOCK_HACK	0x20000000 /* akpm lock hack */
 
 /* Compatibility, for having both ext2_fs.h and ext4_fs.h included at once */
 #ifndef _LINUX_EXT2_FS_H
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 22c4fde..04ab984 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -830,6 +830,8 @@ static int ext4_show_options(struct seq_file *seq, struct vfsmount *vfs)
 		seq_puts(seq, ",journal_async_commit");
 	if (test_opt(sb, NOBH))
 		seq_puts(seq, ",nobh");
+	if (test_opt(sb, AKPM_LOCK_HACK))
+		seq_puts(seq, ",akpm_lock_hack");
 	if (test_opt(sb, I_VERSION))
 		seq_puts(seq, ",i_version");
 	if (!test_opt(sb, DELALLOC))
@@ -1012,7 +1014,7 @@ enum {
 	Opt_ignore, Opt_barrier, Opt_err, Opt_resize, Opt_usrquota,
 	Opt_grpquota, Opt_i_version,
 	Opt_stripe, Opt_delalloc, Opt_nodelalloc,
-	Opt_inode_readahead_blks, Opt_journal_ioprio
+	Opt_inode_readahead_blks, Opt_journal_ioprio, Opt_akpm_lock_hack
 };
 
 static const match_table_t tokens = {
@@ -1072,6 +1074,7 @@ static const match_table_t tokens = {
 	{Opt_nodelalloc, "nodelalloc"},
 	{Opt_inode_readahead_blks, "inode_readahead_blks=%u"},
 	{Opt_journal_ioprio, "journal_ioprio=%u"},
+	{Opt_akpm_lock_hack, "akpm_lock_hack"},
 	{Opt_err, NULL},
 };
 
@@ -1473,6 +1476,9 @@ set_qf_format:
 			*journal_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE,
 							    option);
 			break;
+		case Opt_akpm_lock_hack:
+			set_opt(sbi->s_mount_opt, AKPM_LOCK_HACK);
+			break;
 		default:
 			printk(KERN_ERR
 			       "EXT4-fs: Unrecognized mount option \"%s\" "
@@ -2656,6 +2662,10 @@ static void ext4_init_journal_params(struct super_block *sb, journal_t *journal)
 		journal->j_flags |= JBD2_ABORT_ON_SYNCDATA_ERR;
 	else
 		journal->j_flags &= ~JBD2_ABORT_ON_SYNCDATA_ERR;
+	if (test_opt(sb, AKPM_LOCK_HACK))
+		journal->j_flags |= JBD2_LOCK_HACK;
+	else
+		journal->j_flags &= ~JBD2_LOCK_HACK;
 	spin_unlock(&journal->j_state_lock);
 }
 
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 48c21ba..add3be0 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -549,6 +549,7 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 	int error;
 	char *frozen_buffer = NULL;
 	int need_copy = 0;
+	int locked = 0;
 
 	if (is_handle_aborted(handle))
 		return -EROFS;
@@ -564,7 +565,13 @@ repeat:
 
 	/* @@@ Need to check for errors here at some point. */
 
-	lock_buffer(bh);
+	if (journal->j_flags & JBD2_LOCK_HACK) {
+		if (trylock_buffer(bh))
+			locked = 1;	/* lolz */
+	} else {
+		lock_buffer(bh);
+		locked = 1;
+	}
 	jbd_lock_bh_state(bh);
 
 	/* We now hold the buffer lock so it is safe to query the buffer
@@ -603,7 +610,8 @@ repeat:
 		jbd_unexpected_dirty_buffer(jh);
 	}
 
-	unlock_buffer(bh);
+	if (locked)
+		unlock_buffer(bh);
 
 	error = -EROFS;
 	if (is_handle_aborted(handle)) {
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index adef1c9..4748e5e 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1013,6 +1013,7 @@ struct journal_s
 #define JBD2_ABORT_ON_SYNCDATA_ERR	0x040	/* Abort the journal on file
 						 * data write error in ordered
 						 * mode */
+#define JBD2_LOCK_HACK	0x080	/* akpm's locking hack */
 
 /*
  * Function declarations for the journaling transaction and buffer