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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>
This commit is contained in:
Artem Bityutskiy 2011-05-30 18:19:34 +03:00 committed by Artem Bityutskiy
parent 3766244769
commit 06b282a4cc
1 changed files with 26 additions and 0 deletions
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26
fs/ubifs/tnc_commit.c
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@ -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 */