[PATCH] jbd: do_get_write_access lock contention reduction

We're seeing heavy contention against j_list_lock on 8-way in
do_get_write_access().

We actually don't need j_list_lock in there except for one little case - the
per-bh jbd_lock_bh_state() is sufficient to protect this buffer's internal
state.

On some nice quick LVM array Ram Pai measured an overall 3x speedup from this
patch:

the script took the following time on 265mm1
 real    0m57.504s
 user    0m0.400s
 sys     7m29.867s


 and with the 2patches it took
 real 	0m19.983s
 user    0m0.438s
 sys     1m55.896s

fs/jbd/transaction.c

@@ -522,7 +522,6 @@ static void jbd_unexpected_dirty_buffer(struct journal_head *jh)
  * part of the transaction, that is).
  *
  */
-
 static int
 do_get_write_access(handle_t *handle, struct journal_head *jh,
 			int force_copy, int *credits) 
@@ -550,7 +549,6 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 
 	lock_buffer(bh);
 	jbd_lock_bh_state(bh);
-	spin_lock(&journal->j_list_lock);
 
 	/* We now hold the buffer lock so it is safe to query the buffer
 	 * state.  Is the buffer dirty? 
@@ -566,9 +564,10 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 	 * the buffer dirtied, ugh.)  */
 
 	if (buffer_dirty(bh)) {
-		/* First question: is this buffer already part of the
-		 * current transaction or the existing committing
-		 * transaction? */
+		/*
+		 * 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 || 
@@ -586,22 +585,23 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 
 	error = -EROFS;
 	if (is_handle_aborted(handle)) {
-		spin_unlock(&journal->j_list_lock);
 		jbd_unlock_bh_state(bh);
-		goto out_unlocked;
+		goto out;
 	}
 	error = 0;
 
-	/* The buffer is already part of this transaction if
-	 * b_transaction or b_next_transaction points to it. */
-
+	/*
+	 * 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_locked;
-
-	/* If there is already a copy-out version of this buffer, then
-	 * we don't need to make another one. */
+		goto done;
 
+	/*
+	 * 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);
@@ -611,7 +611,7 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 		handle->h_buffer_credits--;
 		if (credits)
 			(*credits)++;
-		goto done_locked;
+		goto done;
 	}
 
 	/* Is there data here we need to preserve? */
@@ -635,7 +635,6 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 			wait_queue_head_t *wqh;
 
 			JBUFFER_TRACE(jh, "on shadow: sleep");
-			spin_unlock(&journal->j_list_lock);
 			jbd_unlock_bh_state(bh);
 			/* commit wakes up all shadow buffers after IO */
 			wqh = bh_waitq_head(jh2bh(jh));
@@ -661,7 +660,6 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 			JBUFFER_TRACE(jh, "generate frozen data");
 			if (!frozen_buffer) {
 				JBUFFER_TRACE(jh, "allocate memory for buffer");
-				spin_unlock(&journal->j_list_lock);
 				jbd_unlock_bh_state(bh);
 				frozen_buffer = jbd_kmalloc(jh2bh(jh)->b_size,
 							    GFP_NOFS);
@@ -672,12 +670,10 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 					JBUFFER_TRACE(jh, "oom!");
 					error = -ENOMEM;
 					jbd_lock_bh_state(bh);
-					spin_lock(&journal->j_list_lock);
-					goto done_locked;
+					goto done;
 				}
 				goto repeat;
 			}
-
 			jh->b_frozen_data = frozen_buffer;
 			frozen_buffer = NULL;
 			need_copy = 1;
@@ -690,20 +686,22 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 	if (credits)
 		(*credits)++;
 
-	/* Finally, if the buffer is not journaled right now, we need to
-	 * make sure it doesn't get written to disk before the caller
-	 * actually commits the new data. */
-
+	/*
+	 * Finally, if the buffer is not journaled right now, we need to make
+	 * sure it doesn't get written to disk before the caller actually
+	 * commits the new data
+	 */
 	if (!jh->b_transaction) {
 		JBUFFER_TRACE(jh, "no transaction");
 		J_ASSERT_JH(jh, !jh->b_next_transaction);
 		jh->b_transaction = transaction;
 		JBUFFER_TRACE(jh, "file as BJ_Reserved");
+		spin_lock(&journal->j_list_lock);
 		__journal_file_buffer(jh, transaction, BJ_Reserved);
+		spin_unlock(&journal->j_list_lock);
 	}
 
-done_locked:
-	spin_unlock(&journal->j_list_lock);
+done:
 	if (need_copy) {
 		struct page *page;
 		int offset;
@@ -719,11 +717,13 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 	}
 	jbd_unlock_bh_state(bh);
 
-	/* If we are about to journal a buffer, then any revoke pending
-           on it is no longer valid. */
+	/*
+	 * If we are about to journal a buffer, then any revoke pending on it is
+	 * no longer valid
+	 */
 	journal_cancel_revoke(handle, jh);
 
-out_unlocked:
+out:
 	if (frozen_buffer)
 		kfree(frozen_buffer);