bcachefs: journal->buf_lock

Add a new lock for synchronizing between journal IO path and btree write
buffer flush.

Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/journal.c

···
 	static struct lock_class_key res_key;
 	unsigned i;
 
+	mutex_init(&j->buf_lock);
 	spin_lock_init(&j->lock);
 	spin_lock_init(&j->err_lock);
 	init_waitqueue_head(&j->wait);

fs/bcachefs/journal_io.c

···
 	if (ret)
 		goto err;
 
+	mutex_lock(&j->buf_lock);
 	journal_buf_realloc(j, w);
 
 	ret = bch2_journal_write_prep(j, w);
+	mutex_unlock(&j->buf_lock);
 	if (ret)
 		goto err;
 

fs/bcachefs/journal_types.h

···
 	darray_u64		early_journal_entries;
 
 	/*
+	 * Protects journal_buf->data, when accessing without a jorunal
+	 * reservation: for synchronization between the btree write buffer code
+	 * and the journal write path:
+	 */
+	struct mutex		buf_lock;
+	/*
 	 * Two journal entries -- one is currently open for new entries, the
 	 * other is possibly being written out.
 	 */