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authorArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2011-05-30 18:19:34 +0300
committerArtem Bityutskiy <dedekind1@gmail.com>2011-07-04 10:54:27 +0300
commit06b282a4cc02f37414c14c94a2f154ca250cf73f (patch)
tree058646506c1e2652de8f9d59d56f564d3b5e66b9 /fs/ubifs
parent376624476921e43d8b87498161a2ffba6ab8d5aa (diff)
downloadlinux-06b282a4cc02f37414c14c94a2f154ca250cf73f.tar.bz2
UBIFS: add few commentaries about TNC
Add a couple of comments - while looking into TNC I could not easily figure out few facts, so it is a good idea to document them in the code. Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Diffstat (limited to 'fs/ubifs')
-rw-r--r--fs/ubifs/tnc_commit.c26
1 files changed, 26 insertions, 0 deletions
diff --git a/fs/ubifs/tnc_commit.c b/fs/ubifs/tnc_commit.c
index f50c3e5da263..d6fab1a9986c 100644
--- a/fs/ubifs/tnc_commit.c
+++ b/fs/ubifs/tnc_commit.c
@@ -89,6 +89,10 @@ static int make_idx_node(struct ubifs_info *c, struct ubifs_idx_node *idx,
ubifs_assert(ubifs_zn_dirty(znode));
ubifs_assert(ubifs_zn_cow(znode));
+ /*
+ * Note, unlike 'write_index()' we do not add memory barriers here
+ * because this function is called with @c->tnc_mutex locked.
+ */
__clear_bit(DIRTY_ZNODE, &znode->flags);
__clear_bit(COW_ZNODE, &znode->flags);
@@ -903,6 +907,28 @@ static int write_index(struct ubifs_info *c)
clear_bit(COW_ZNODE, &znode->flags);
smp_mb__after_clear_bit();
+ /*
+ * We have marked the znode as clean but have not updated the
+ * @c->clean_zn_cnt counter. If this znode becomes dirty again
+ * before 'free_obsolete_znodes()' is called, then
+ * @c->clean_zn_cnt will be decremented before it gets
+ * incremented (resulting in 2 decrements for the same znode).
+ * This means that @c->clean_zn_cnt may become negative for a
+ * while.
+ *
+ * Q: why we cannot increment @c->clean_zn_cnt?
+ * A: because we do not have the @c->tnc_mutex locked, and the
+ * following code would be racy and buggy:
+ *
+ * if (!ubifs_zn_obsolete(znode)) {
+ * atomic_long_inc(&c->clean_zn_cnt);
+ * atomic_long_inc(&ubifs_clean_zn_cnt);
+ * }
+ *
+ * Thus, we just delay the @c->clean_zn_cnt update until we
+ * have the mutex locked.
+ */
+
/* Do not access znode from this point on */
/* Update buffer positions */