static void
__journal_try_to_free_buffer(journal_t *journal, struct buffer_head *bh)
{
	struct journal_head *jh;

	jh = bh2jh(bh);

	if (buffer_locked(bh) || buffer_dirty(bh))
		goto out;

	if (jh->b_next_transaction != NULL)
		goto out;

	spin_lock(&journal->j_list_lock);
	if (jh->b_transaction != NULL && jh->b_cp_transaction == NULL) {
		if (jh->b_jlist == BJ_SyncData || jh->b_jlist == BJ_Locked) {
			/* A written-back ordered data buffer */
			JBUFFER_TRACE(jh, "release data");
			__journal_unfile_buffer(jh);
			journal_remove_journal_head(bh);
			__brelse(bh);
		}
	} else if (jh->b_cp_transaction != NULL && jh->b_transaction == NULL) {
		/* written-back checkpointed metadata buffer */
		if (jh->b_jlist == BJ_None) {
			JBUFFER_TRACE(jh, "remove from checkpoint list");
			__journal_remove_checkpoint(jh);
			journal_remove_journal_head(bh);
			__brelse(bh);
		}
	}
	spin_unlock(&journal->j_list_lock);
out:
	return;
}

/*
 * journal_remove_checkpoint: called after a buffer has been committed
 * to disk (either by being write-back flushed to disk, or being
 * committed to the log).
 *
 * We cannot safely clean a transaction out of the log until all of the
 * buffer updates committed in that transaction have safely been stored
 * elsewhere on disk.  To achieve this, all of the buffers in a
 * transaction need to be maintained on the transaction's checkpoint
 * lists until they have been rewritten, at which point this function is
 * called to remove the buffer from the existing transaction's
 * checkpoint lists.
 *
 * The function returns 1 if it frees the transaction, 0 otherwise.
 * The function can free jh and bh.
 *
 * This function is called with j_list_lock held.
 */
int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
{
	struct transaction_chp_stats_s *stats;
	transaction_t *transaction;
	journal_t *journal;
	int ret = 0;

	JBUFFER_TRACE(jh, "entry");

	if ((transaction = jh->b_cp_transaction) == NULL) {
		JBUFFER_TRACE(jh, "not on transaction");
		goto out;
	}
	journal = transaction->t_journal;

	JBUFFER_TRACE(jh, "removing from transaction");
	__buffer_unlink(jh);
	jh->b_cp_transaction = NULL;
	jbd2_journal_put_journal_head(jh);

	if (transaction->t_checkpoint_list != NULL ||
	    transaction->t_checkpoint_io_list != NULL)
		goto out;

	/*
	 * There is one special case to worry about: if we have just pulled the
	 * buffer off a running or committing transaction's checkpoing list,
	 * then even if the checkpoint list is empty, the transaction obviously
	 * cannot be dropped!
	 *
	 * The locking here around t_state is a bit sleazy.
	 * See the comment at the end of jbd2_journal_commit_transaction().
	 */
	if (transaction->t_state != T_FINISHED)
		goto out;

	/* OK, that was the last buffer for the transaction: we can now
	   safely remove this transaction from the log */
	stats = &transaction->t_chp_stats;
	if (stats->cs_chp_time)
		stats->cs_chp_time = jbd2_time_diff(stats->cs_chp_time,
						    jiffies);
	trace_jbd2_checkpoint_stats(journal->j_fs_dev->bd_dev,
				    transaction->t_tid, stats);

	__jbd2_journal_drop_transaction(journal, transaction);

	if ((journal->j_commit_callback == NULL) || (transaction->t_callbacked)) {
		jbd2_journal_free_transaction(transaction);
	} else {
		transaction->t_dropped = 1;
	}

	/* Just in case anybody was waiting for more transactions to be
           checkpointed... */
	wake_up(&journal->j_wait_logspace);
	ret = 1;
out:
	return ret;
}

int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
{
	struct transaction_chp_stats_s *stats;
	transaction_t *transaction;
	journal_t *journal;
	int ret = 0;

	JBUFFER_TRACE(jh, "entry");

	if ((transaction = jh->b_cp_transaction) == NULL) {
		JBUFFER_TRACE(jh, "not on transaction");
		goto out;
	}
	journal = transaction->t_journal;

	JBUFFER_TRACE(jh, "removing from transaction");
	__buffer_unlink(jh);
	jh->b_cp_transaction = NULL;
	jbd2_journal_put_journal_head(jh);

	if (transaction->t_checkpoint_list != NULL ||
	    transaction->t_checkpoint_io_list != NULL)
		goto out;

	/*
                                                                        
                                                                     
                                                                        
                      
   
                                                    
                                                                    
  */
	if (transaction->t_state != T_FINISHED)
		goto out;

	/*                                                             
                                                */
	stats = &transaction->t_chp_stats;
	if (stats->cs_chp_time)
		stats->cs_chp_time = jbd2_time_diff(stats->cs_chp_time,
						    jiffies);
	trace_jbd2_checkpoint_stats(journal->j_fs_dev->bd_dev,
				    transaction->t_tid, stats);

	__jbd2_journal_drop_transaction(journal, transaction);
	jbd2_journal_free_transaction(transaction);

	/*                                                             
                           */
	wake_up(&journal->j_wait_logspace);
	ret = 1;
out:
	return ret;
}

int journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
{
	int err;
	struct journal_head *jh = journal_add_journal_head(bh);
	char *committed_data = NULL;

	JBUFFER_TRACE(jh, "entry");

	/*
	 * Do this first --- it can drop the journal lock, so we want to
	 * make sure that obtaining the committed_data is done
	 * atomically wrt. completion of any outstanding commits.
	 */
	err = do_get_write_access(handle, jh, 1);
	if (err)
		goto out;

repeat:
	if (!jh->b_committed_data) {
		committed_data = jbd_alloc(jh2bh(jh)->b_size, GFP_NOFS);
		if (!committed_data) {
			printk(KERN_EMERG "%s: No memory for committed data\n",
				__func__);
			err = -ENOMEM;
			goto out;
		}
	}

	jbd_lock_bh_state(bh);
	if (!jh->b_committed_data) {
		/* Copy out the current buffer contents into the
		 * preserved, committed copy. */
		JBUFFER_TRACE(jh, "generate b_committed data");
		if (!committed_data) {
			jbd_unlock_bh_state(bh);
			goto repeat;
		}

		jh->b_committed_data = committed_data;
		committed_data = NULL;
		memcpy(jh->b_committed_data, bh->b_data, bh->b_size);
	}
	jbd_unlock_bh_state(bh);
out:
	journal_put_journal_head(jh);
	if (unlikely(committed_data))
		jbd_free(committed_data, bh->b_size);
	return err;
}

int __journal_remove_checkpoint(struct journal_head *jh)
{
	transaction_t *transaction;
	journal_t *journal;
	int ret = 0;

	JBUFFER_TRACE(jh, "entry");

	if ((transaction = jh->b_cp_transaction) == NULL) {
		JBUFFER_TRACE(jh, "not on transaction");
		goto out;
	}
	journal = transaction->t_journal;

	__buffer_unlink(jh);
	jh->b_cp_transaction = NULL;

	if (transaction->t_checkpoint_list != NULL ||
	    transaction->t_checkpoint_io_list != NULL)
		goto out;
	JBUFFER_TRACE(jh, "transaction has no more buffers");

	/*
	 * There is one special case to worry about: if we have just pulled the
	 * buffer off a committing transaction's forget list, then even if the
	 * checkpoint list is empty, the transaction obviously cannot be
	 * dropped!
	 *
	 * The locking here around j_committing_transaction is a bit sleazy.
	 * See the comment at the end of journal_commit_transaction().
	 */
	if (transaction == journal->j_committing_transaction) {
		JBUFFER_TRACE(jh, "belongs to committing transaction");
		goto out;
	}

	/* OK, that was the last buffer for the transaction: we can now
	   safely remove this transaction from the log */

	__journal_drop_transaction(journal, transaction);

	/* Just in case anybody was waiting for more transactions to be
           checkpointed... */
	wake_up(&journal->j_wait_logspace);
	ret = 1;
out:
	JBUFFER_TRACE(jh, "exit");
	return ret;
}

/* Done it all: now write the commit record.  We should have
 * cleaned up our previous buffers by now, so if we are in abort
 * mode we can now just skip the rest of the journal write
 * entirely.
 *
 * Returns 1 if the journal needs to be aborted or 0 on success
 */
static int journal_write_commit_record(journal_t *journal,
					transaction_t *commit_transaction)
{
	struct journal_head *descriptor;
	struct buffer_head *bh;
	journal_header_t *header;
	int ret;

	if (is_journal_aborted(journal))
		return 0;

	descriptor = journal_get_descriptor_buffer(journal);
	if (!descriptor)
		return 1;

	bh = jh2bh(descriptor);

	header = (journal_header_t *)(bh->b_data);
	header->h_magic = cpu_to_be32(JFS_MAGIC_NUMBER);
	header->h_blocktype = cpu_to_be32(JFS_COMMIT_BLOCK);
	header->h_sequence = cpu_to_be32(commit_transaction->t_tid);

	JBUFFER_TRACE(descriptor, "write commit block");
	set_buffer_dirty(bh);

	if (journal->j_flags & JFS_BARRIER)
		ret = __sync_dirty_buffer(bh, WRITE_SYNC | WRITE_FLUSH_FUA);
	else
		ret = sync_dirty_buffer(bh);

	put_bh(bh);		/* One for getblk() */
	journal_put_journal_head(descriptor);

	return (ret == -EIO);
}

int jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
{
	transaction_t *transaction = handle->h_transaction;
	journal_t *journal = transaction->t_journal;
	struct journal_head *jh = jbd2_journal_add_journal_head(bh);
	int err;

	jbd_debug(5, "journal_head %p\n", jh);
	err = -EROFS;
	if (is_handle_aborted(handle))
		goto out;
	err = 0;

	JBUFFER_TRACE(jh, "entry");
	jbd_lock_bh_state(bh);
	spin_lock(&journal->j_list_lock);
	J_ASSERT_JH(jh, (jh->b_transaction == transaction ||
		jh->b_transaction == NULL ||
		(jh->b_transaction == journal->j_committing_transaction &&
			  jh->b_jlist == BJ_Forget)));

	J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
	J_ASSERT_JH(jh, buffer_locked(jh2bh(jh)));

	if (jh->b_transaction == NULL) {
		clear_buffer_dirty(jh2bh(jh));
		
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "file as BJ_Reserved");
		__jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
	} else if (jh->b_transaction == journal->j_committing_transaction) {
		
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "set next transaction");
		jh->b_next_transaction = transaction;
	}
	spin_unlock(&journal->j_list_lock);
	jbd_unlock_bh_state(bh);

	JBUFFER_TRACE(jh, "cancelling revoke");
	jbd2_journal_cancel_revoke(handle, jh);
out:
	jbd2_journal_put_journal_head(jh);
	return err;
}

/* Done it all: now write the commit record.  We should have
 * cleaned up our previous buffers by now, so if we are in abort
 * mode we can now just skip the rest of the journal write
 * entirely.
 *
 * Returns 1 if the journal needs to be aborted or 0 on success
 */
static int journal_write_commit_record(journal_t *journal,
					transaction_t *commit_transaction)
{
	struct journal_head *descriptor;
	struct buffer_head *bh;
	int i, ret;
	int barrier_done = 0;

	if (is_journal_aborted(journal))
		return 0;

	descriptor = jbd2_journal_get_descriptor_buffer(journal);
	if (!descriptor)
		return 1;

	bh = jh2bh(descriptor);

	/* AKPM: buglet - add `i' to tmp! */
	for (i = 0; i < bh->b_size; i += 512) {
		journal_header_t *tmp = (journal_header_t*)bh->b_data;
		tmp->h_magic = cpu_to_be32(JBD2_MAGIC_NUMBER);
		tmp->h_blocktype = cpu_to_be32(JBD2_COMMIT_BLOCK);
		tmp->h_sequence = cpu_to_be32(commit_transaction->t_tid);
	}

	JBUFFER_TRACE(descriptor, "write commit block");
	set_buffer_dirty(bh);
	if (journal->j_flags & JBD2_BARRIER) {
		set_buffer_ordered(bh);
		barrier_done = 1;
	}
	ret = sync_dirty_buffer(bh);
	/* is it possible for another commit to fail at roughly
	 * the same time as this one?  If so, we don't want to
	 * trust the barrier flag in the super, but instead want
	 * to remember if we sent a barrier request
	 */
	if (ret == -EOPNOTSUPP && barrier_done) {
		char b[BDEVNAME_SIZE];

		printk(KERN_WARNING
			"JBD: barrier-based sync failed on %s - "
			"disabling barriers\n",
			bdevname(journal->j_dev, b));
		spin_lock(&journal->j_state_lock);
		journal->j_flags &= ~JBD2_BARRIER;
		spin_unlock(&journal->j_state_lock);

		/* And try again, without the barrier */
		clear_buffer_ordered(bh);
		set_buffer_uptodate(bh);
		set_buffer_dirty(bh);
		ret = sync_dirty_buffer(bh);
	}
	put_bh(bh);		/* One for getblk() */
	jbd2_journal_put_journal_head(descriptor);

	return (ret == -EIO);
}

/* Done it all: now write the commit record.  We should have
 * cleaned up our previous buffers by now, so if we are in abort
 * mode we can now just skip the rest of the journal write
 * entirely.
 *
 * Returns 1 if the journal needs to be aborted or 0 on success
 */
static int journal_write_commit_record(journal_t *journal,
					transaction_t *commit_transaction)
{
	struct journal_head *descriptor;
	struct buffer_head *bh;
	journal_header_t *header;
	int ret;

	if (is_journal_aborted(journal))
		return 0;

	descriptor = journal_get_descriptor_buffer(journal);
	if (!descriptor)
		return 1;

	bh = jh2bh(descriptor);

	header = (journal_header_t *)(bh->b_data);
	header->h_magic = cpu_to_be32(JFS_MAGIC_NUMBER);
	header->h_blocktype = cpu_to_be32(JFS_COMMIT_BLOCK);
	header->h_sequence = cpu_to_be32(commit_transaction->t_tid);

	JBUFFER_TRACE(descriptor, "write commit block");
	set_buffer_dirty(bh);

	if (journal->j_flags & JFS_BARRIER) {
		ret = __sync_dirty_buffer(bh, WRITE_SYNC | WRITE_BARRIER);

		/*
		 * Is it possible for another commit to fail at roughly
		 * the same time as this one?  If so, we don't want to
		 * trust the barrier flag in the super, but instead want
		 * to remember if we sent a barrier request
		 */
		if (ret == -EOPNOTSUPP) {
			char b[BDEVNAME_SIZE];

			printk(KERN_WARNING
				"JBD: barrier-based sync failed on %s - "
				"disabling barriers\n",
				bdevname(journal->j_dev, b));
			spin_lock(&journal->j_state_lock);
			journal->j_flags &= ~JFS_BARRIER;
			spin_unlock(&journal->j_state_lock);

			/* And try again, without the barrier */
			set_buffer_uptodate(bh);
			set_buffer_dirty(bh);
			ret = sync_dirty_buffer(bh);
		}
	} else {
		ret = sync_dirty_buffer(bh);
	}

	put_bh(bh);		/* One for getblk() */
	journal_put_journal_head(descriptor);

	return (ret == -EIO);
}

static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
{
	int may_free = 1;
	struct buffer_head *bh = jh2bh(jh);

	if (jh->b_cp_transaction) {
		JBUFFER_TRACE(jh, "on running+cp transaction");
		__jbd2_journal_temp_unlink_buffer(jh);
		clear_buffer_dirty(bh);
		__jbd2_journal_file_buffer(jh, transaction, BJ_Forget);
		may_free = 0;
	} else {
		JBUFFER_TRACE(jh, "on running transaction");
		__jbd2_journal_unfile_buffer(jh);
	}
	return may_free;
}

static void write_one_revoke_record(journal_t *journal,
				    transaction_t *transaction,
				    struct journal_head **descriptorp,
				    int *offsetp,
				    struct jbd2_revoke_record_s *record)
{
	struct journal_head *descriptor;
	int offset;
	journal_header_t *header;

	/* If we are already aborting, this all becomes a noop.  We
           still need to go round the loop in
           jbd2_journal_write_revoke_records in order to free all of the
           revoke records: only the IO to the journal is omitted. */
	if (is_journal_aborted(journal))
		return;

	descriptor = *descriptorp;
	offset = *offsetp;

	/* Make sure we have a descriptor with space left for the record */
	if (descriptor) {
		if (offset == journal->j_blocksize) {
			flush_descriptor(journal, descriptor, offset);
			descriptor = NULL;
		}
	}

	if (!descriptor) {
		descriptor = jbd2_journal_get_descriptor_buffer(journal);
		if (!descriptor)
			return;
		header = (journal_header_t *) &jh2bh(descriptor)->b_data[0];
		header->h_magic     = cpu_to_be32(JBD2_MAGIC_NUMBER);
		header->h_blocktype = cpu_to_be32(JBD2_REVOKE_BLOCK);
		header->h_sequence  = cpu_to_be32(transaction->t_tid);

		/* Record it so that we can wait for IO completion later */
		JBUFFER_TRACE(descriptor, "file as BJ_LogCtl");
		jbd2_journal_file_buffer(descriptor, transaction, BJ_LogCtl);

		offset = sizeof(jbd2_journal_revoke_header_t);
		*descriptorp = descriptor;
	}

	if (JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_64BIT)) {
		* ((__be64 *)(&jh2bh(descriptor)->b_data[offset])) =
			cpu_to_be64(record->blocknr);
		offset += 8;

	} else {
		* ((__be32 *)(&jh2bh(descriptor)->b_data[offset])) =
			cpu_to_be32(record->blocknr);
		offset += 4;
	}

	*offsetp = offset;
}

int jbd2_journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
{
	int err;
	struct journal_head *jh = jbd2_journal_add_journal_head(bh);
	char *committed_data = NULL;

	JBUFFER_TRACE(jh, "entry");

	err = do_get_write_access(handle, jh, 1);
	if (err)
		goto out;

repeat:
	if (!jh->b_committed_data) {
		committed_data = jbd2_alloc(jh2bh(jh)->b_size, GFP_NOFS);
		if (!committed_data) {
			printk(KERN_EMERG "%s: No memory for committed data\n",
				__func__);
			err = -ENOMEM;
			goto out;
		}
	}

	jbd_lock_bh_state(bh);
	if (!jh->b_committed_data) {
		JBUFFER_TRACE(jh, "generate b_committed data");
		if (!committed_data) {
			jbd_unlock_bh_state(bh);
			goto repeat;
		}

		jh->b_committed_data = committed_data;
		committed_data = NULL;
		memcpy(jh->b_committed_data, bh->b_data, bh->b_size);
	}
	jbd_unlock_bh_state(bh);
out:
	jbd2_journal_put_journal_head(jh);
	if (unlikely(committed_data))
		jbd2_free(committed_data, bh->b_size);
	return err;
}

/*
 * Try to release a checkpointed buffer from its transaction.
 * Returns 1 if we released it and 2 if we also released the
 * whole transaction.
 *
 * Requires j_list_lock
 */
static int __try_to_free_cp_buf(struct journal_head *jh)
{
	int ret = 0;
	struct buffer_head *bh = jh2bh(jh);

	if (jh->b_transaction == NULL && !buffer_locked(bh) &&
	    !buffer_dirty(bh) && !buffer_write_io_error(bh)) {
		JBUFFER_TRACE(jh, "remove from checkpoint list");
		ret = __jbd2_journal_remove_checkpoint(jh) + 1;
	}
	return ret;
}

static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
{
	int may_free = 1;
	struct buffer_head *bh = jh2bh(jh);

	__journal_unfile_buffer(jh);

	if (jh->b_cp_transaction) {
		JBUFFER_TRACE(jh, "on running+cp transaction");
		/*
		 * We don't want to write the buffer anymore, clear the
		 * bit so that we don't confuse checks in
		 * __journal_file_buffer
		 */
		clear_buffer_dirty(bh);
		__journal_file_buffer(jh, transaction, BJ_Forget);
		may_free = 0;
	} else {
		JBUFFER_TRACE(jh, "on running transaction");
		journal_remove_journal_head(bh);
		__brelse(bh);
	}
	return may_free;
}

/*
 * Try to release a checkpointed buffer from its transaction.
 * Returns 1 if we released it.
 * Requires journal_datalist_lock
 */
static int __try_to_free_cp_buf(struct journal_head *jh)
{
	int ret = 0;
	struct buffer_head *bh = jh2bh(jh);

	if (jh->b_jlist == BJ_None && !buffer_locked(bh) && !buffer_dirty(bh)) {
		JBUFFER_TRACE(jh, "remove from checkpoint list");
		__journal_remove_checkpoint(jh);
		__journal_remove_journal_head(bh);
		BUFFER_TRACE(bh, "release");
		/* BUF_LOCKED -> BUF_CLEAN (fwiw) */
		refile_buffer(bh);
		__brelse(bh);
		ret = 1;
	}
	return ret;
}

/*
 * Try to release a checkpointed buffer from its transaction.
 * Returns 1 if we released it and 2 if we also released the
 * whole transaction.
 *
 * Requires j_list_lock
 * Called under jbd_lock_bh_state(jh2bh(jh)), and drops it
 */
static int __try_to_free_cp_buf(struct journal_head *jh)
{
	int ret = 0;
	struct buffer_head *bh = jh2bh(jh);

	if (jh->b_jlist == BJ_None && !buffer_locked(bh) && !buffer_dirty(bh)) {
		JBUFFER_TRACE(jh, "remove from checkpoint list");
		ret = __jbd2_journal_remove_checkpoint(jh) + 1;
		jbd_unlock_bh_state(bh);
		jbd2_journal_remove_journal_head(bh);
		BUFFER_TRACE(bh, "release");
		__brelse(bh);
	} else {
		jbd_unlock_bh_state(bh);
	}
	return ret;
}

/*
 * journal_insert_checkpoint: put a committed buffer onto a checkpoint
 * list so that we know when it is safe to clean the transaction out of
 * the log.
 *
 * Called with the journal locked.
 * Called with j_list_lock held.
 */
void __journal_insert_checkpoint(struct journal_head *jh, 
			       transaction_t *transaction)
{
	JBUFFER_TRACE(jh, "entry");
	J_ASSERT_JH(jh, buffer_dirty(jh2bh(jh)) || buffer_jbddirty(jh2bh(jh)));
	J_ASSERT_JH(jh, jh->b_cp_transaction == NULL);

	jh->b_cp_transaction = transaction;

	if (!transaction->t_checkpoint_list) {
		jh->b_cpnext = jh->b_cpprev = jh;
	} else {
		jh->b_cpnext = transaction->t_checkpoint_list;
		jh->b_cpprev = transaction->t_checkpoint_list->b_cpprev;
		jh->b_cpprev->b_cpnext = jh;
		jh->b_cpnext->b_cpprev = jh;
	}
	transaction->t_checkpoint_list = jh;
}

static void
__journal_try_to_free_buffer(journal_t *journal, struct buffer_head *bh)
{
	struct journal_head *jh;

	jh = bh2jh(bh);

	if (buffer_locked(bh) || buffer_dirty(bh))
		goto out;

	if (jh->b_next_transaction != NULL)
		goto out;

	spin_lock(&journal->j_list_lock);
	if (jh->b_cp_transaction != NULL && jh->b_transaction == NULL) {
		
		JBUFFER_TRACE(jh, "remove from checkpoint list");
		__jbd2_journal_remove_checkpoint(jh);
	}
	spin_unlock(&journal->j_list_lock);
out:
	return;
}

int journal_get_create_access(handle_t *handle, struct buffer_head *bh)
{
	transaction_t *transaction = handle->h_transaction;
	journal_t *journal = transaction->t_journal;
	struct journal_head *jh = journal_add_journal_head(bh);
	int err;

	jbd_debug(5, "journal_head %p\n", jh);
	err = -EROFS;
	if (is_handle_aborted(handle))
		goto out;
	err = 0;

	JBUFFER_TRACE(jh, "entry");
	/*
	 * The buffer may already belong to this transaction due to pre-zeroing
	 * in the filesystem's new_block code.  It may also be on the previous,
	 * committing transaction's lists, but it HAS to be in Forget state in
	 * that case: the transaction must have deleted the buffer for it to be
	 * reused here.
	 */
	jbd_lock_bh_state(bh);
	spin_lock(&journal->j_list_lock);
	J_ASSERT_JH(jh, (jh->b_transaction == transaction ||
		jh->b_transaction == NULL ||
		(jh->b_transaction == journal->j_committing_transaction &&
			  jh->b_jlist == BJ_Forget)));

	J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
	J_ASSERT_JH(jh, buffer_locked(jh2bh(jh)));

	if (jh->b_transaction == NULL) {
		/*
		 * Previous journal_forget() could have left the buffer
		 * with jbddirty bit set because it was being committed. When
		 * the commit finished, we've filed the buffer for
		 * checkpointing and marked it dirty. Now we are reallocating
		 * the buffer so the transaction freeing it must have
		 * committed and so it's safe to clear the dirty bit.
		 */
		clear_buffer_dirty(jh2bh(jh));
		jh->b_transaction = transaction;

		/* first access by this transaction */
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "file as BJ_Reserved");
		__journal_file_buffer(jh, transaction, BJ_Reserved);
	} else if (jh->b_transaction == journal->j_committing_transaction) {
		/* first access by this transaction */
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "set next transaction");
		jh->b_next_transaction = transaction;
	}
	spin_unlock(&journal->j_list_lock);
	jbd_unlock_bh_state(bh);

	/*
	 * akpm: I added this.  ext3_alloc_branch can pick up new indirect
	 * blocks which contain freed but then revoked metadata.  We need
	 * to cancel the revoke in case we end up freeing it yet again
	 * and the reallocating as data - this would cause a second revoke,
	 * which hits an assertion error.
	 */
	JBUFFER_TRACE(jh, "cancelling revoke");
	journal_cancel_revoke(handle, jh);
	journal_put_journal_head(jh);
out:
	return err;
}

static int
do_get_write_access(handle_t *handle, struct journal_head *jh,
			int force_copy)
{
	struct buffer_head *bh;
	transaction_t *transaction;
	journal_t *journal;
	int error;
	char *frozen_buffer = NULL;
	int need_copy = 0;

	if (is_handle_aborted(handle))
		return -EROFS;

	transaction = handle->h_transaction;
	journal = transaction->t_journal;

	jbd_debug(5, "buffer_head %p, force_copy %d\n", jh, force_copy);

	JBUFFER_TRACE(jh, "entry");
repeat:
	bh = jh2bh(jh);

	/* @@@ Need to check for errors here at some point. */

	lock_buffer(bh);
	jbd_lock_bh_state(bh);

	/* We now hold the buffer lock so it is safe to query the buffer
	 * state.  Is the buffer dirty?
	 *
	 * If so, there are two possibilities.  The buffer may be
	 * non-journaled, and undergoing a quite legitimate writeback.
	 * Otherwise, it is journaled, and we don't expect dirty buffers
	 * in that state (the buffers should be marked JBD_Dirty
	 * instead.)  So either the IO is being done under our own
	 * control and this is a bug, or it's a third party IO such as
	 * dump(8) (which may leave the buffer scheduled for read ---
	 * ie. locked but not dirty) or tune2fs (which may actually have
	 * the buffer dirtied, ugh.)  */

	if (buffer_dirty(bh)) {
		/*
		 * First question: is this buffer already part of the current
		 * transaction or the existing committing transaction?
		 */
		if (jh->b_transaction) {
			J_ASSERT_JH(jh,
				jh->b_transaction == transaction ||
				jh->b_transaction ==
					journal->j_committing_transaction);
			if (jh->b_next_transaction)
				J_ASSERT_JH(jh, jh->b_next_transaction ==
							transaction);
			warn_dirty_buffer(bh);
		}
		/*
		 * In any case we need to clean the dirty flag and we must
		 * do it under the buffer lock to be sure we don't race
		 * with running write-out.
		 */
		JBUFFER_TRACE(jh, "Journalling dirty buffer");
		clear_buffer_dirty(bh);
		set_buffer_jbddirty(bh);
	}

	unlock_buffer(bh);

	error = -EROFS;
	if (is_handle_aborted(handle)) {
		jbd_unlock_bh_state(bh);
		goto out;
	}
	error = 0;

	/*
	 * The buffer is already part of this transaction if b_transaction or
	 * b_next_transaction points to it
	 */
	if (jh->b_transaction == transaction ||
	    jh->b_next_transaction == transaction)
		goto done;

	/*
	 * this is the first time this transaction is touching this buffer,
	 * reset the modified flag
	 */
	jh->b_modified = 0;

	/*
	 * If there is already a copy-out version of this buffer, then we don't
	 * need to make another one
	 */
	if (jh->b_frozen_data) {
		JBUFFER_TRACE(jh, "has frozen data");
		J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
		jh->b_next_transaction = transaction;
		goto done;
	}

//.........这里部分代码省略.........