Re: linux-next: manual merge of the akpm tree with Linus' tree

Linus Torvalds
2013-09-13T20:25:51Z

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On Fri, Sep 13, 2013 at 4:00 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
\>
> It is right - for one thing, we are holding the lock on that LRU list,
> so list_lru_del() would deadlock right there. For another, the same
> list_lru_walk (OK, list_lru_walk_node()) will do ->nr_items decrement
> when we return LRU_REMOVED to it, so we don't want to do it twice.
> Plain list_del_init() is correct here.

Yes. And I found the opposite bug in one place: when we are collecting
dentries by walking the parents etc, we do *not* hold the global RCU
lock, so we cannot use the "d_lru_shrink_list()" thing after all. It's
correct as far as the internal logic of fs/dcache.c goes, but it
violates the global LRU list rules. So I replaced that with a
dentry_lru_del() followed by a d_shrink_add() instead.

Updated patch attached.

Linus

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fs/dcache.c | 129 +++++++++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 102 insertions(+), 27 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 1bd4614ce93b..435b97560674 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -357,15 +357,80 @@ static void dentry_unlink_inode(struct dentry * dentry)
}

/*
+ * The DCACHE_LRU_LIST bit is set whenever the 'd_lru' entry
+ * is in use - which includes both the "real" per-superblock
+ * LRU list _and_ the DCACHE_SHRINK_LIST use.
+ *
+ * The DCACHE_SHRINK_LIST bit is set whenever the dentry is
+ * on the shrink list (ie not on the superblock LRU list).
+ *
+ * The per-cpu "nr_dentry_unused" counters are updated with
+ * the DCACHE_LRU_LIST bit.
+ *
+ * These helper functions make sure we always follow the
+ * rules. d_lock must be held by the caller.
+ */
+#define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
+static void d_lru_add(struct dentry *dentry)
+{
+ D_FLAG_VERIFY(dentry, 0);
+ dentry->d_flags |= DCACHE_LRU_LIST;
+ this_cpu_inc(nr_dentry_unused);
+ WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
+}
+
+static void d_lru_del(struct dentry *dentry)
+{
+ D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+ dentry->d_flags &= ~DCACHE_LRU_LIST;
+ this_cpu_dec(nr_dentry_unused);
+ WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
+}
+
+static void d_shrink_del(struct dentry *dentry)
+{
+ D_FLAG_VERIFY(dentry, DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
+ list_del_init(&dentry->d_lru);
+ dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
+ this_cpu_dec(nr_dentry_unused);
+}
+
+static void d_shrink_add(struct dentry *dentry, struct list_head *list)
+{
+ D_FLAG_VERIFY(dentry, 0);
+ list_add(&dentry->d_lru, list);
+ dentry->d_flags |= DCACHE_SHRINK_LIST | DCACHE_LRU_LIST;
+ this_cpu_inc(nr_dentry_unused);
+}
+
+/*
+ * These can only be called under the global LRU lock, ie during the
+ * callback for freeing the LRU list. "isolate" removes it from the
+ * LRU lists entirely, while shrink_move moves it to the indicated
+ * private list.
+ */
+static void d_lru_isolate(struct dentry *dentry)
+{
+ D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+ dentry->d_flags &= ~DCACHE_LRU_LIST;
+ this_cpu_dec(nr_dentry_unused);
+ list_del_init(&dentry->d_lru);
+}
+
+static void d_lru_shrink_move(struct dentry *dentry, struct list_head *list)
+{
+ D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+ dentry->d_flags |= DCACHE_SHRINK_LIST;
+ list_move_tail(&dentry->d_lru, list);
+}
+
+/*
* dentry_lru_(add|del)_list) must be called with d_lock held.
*/
static void dentry_lru_add(struct dentry *dentry)
{
- if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST))) {
- if (list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru))
- this_cpu_inc(nr_dentry_unused);
- dentry->d_flags |= DCACHE_LRU_LIST;
- }
+ if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST)))
+ d_lru_add(dentry);
}

/*
@@ -377,15 +442,11 @@ static void dentry_lru_add(struct dentry *dentry)
*/
static void dentry_lru_del(struct dentry *dentry)
{
- if (dentry->d_flags & DCACHE_SHRINK_LIST) {
- list_del_init(&dentry->d_lru);
- dentry->d_flags &= ~DCACHE_SHRINK_LIST;
- return;
+ if (dentry->d_flags & DCACHE_LRU_LIST) {
+ if (dentry->d_flags & DCACHE_SHRINK_LIST)
+ return d_shrink_del(dentry);
+ d_lru_del(dentry);
}
-
- if (list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru))
- this_cpu_dec(nr_dentry_unused);
- dentry->d_flags &= ~DCACHE_LRU_LIST;
}

/**
@@ -837,6 +898,13 @@ static void shrink_dentry_list(struct list_head *list)
dentry = list_entry_rcu(list->prev, struct dentry, d_lru);
if (&dentry->d_lru == list)
break; /* empty */
+
+
+ /*
+ * Get the dentry lock, and re-verify that the dentry is
+ * this on the shrinking list. If it is, we know that
+ * DCACHE_SHRINK_LIST and DCACHE_LRU_LIST are set.
+ */
spin_lock(&dentry->d_lock);
if (dentry != list_entry(list->prev, struct dentry, d_lru)) {
spin_unlock(&dentry->d_lock);
@@ -848,8 +916,7 @@ static void shrink_dentry_list(struct list_head *list)
* to the LRU here, so we can simply remove it from the list
* here regardless of whether it is referenced or not.
*/
- list_del_init(&dentry->d_lru);
- dentry->d_flags &= ~DCACHE_SHRINK_LIST;
+ d_shrink_del(dentry);

/*
* We found an inuse dentry which was not removed from
@@ -861,12 +928,20 @@ static void shrink_dentry_list(struct list_head *list)
}
rcu_read_unlock();

+ /*
+ * If 'try_to_prune()' returns a dentry, it will
+ * be the same one we passed in, and d_lock will
+ * have been held the whole time, so it will not
+ * have been added to any other lists. We failed
+ * to get the inode lock.
+ *
+ * We just add it back to the shrink list.
+ */
dentry = try_prune_one_dentry(dentry);

rcu_read_lock();
if (dentry) {
- dentry->d_flags |= DCACHE_SHRINK_LIST;
- list_add(&dentry->d_lru, list);
+ d_shrink_add(dentry, list);
spin_unlock(&dentry->d_lock);
}
}
@@ -894,7 +969,7 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
* another pass through the LRU.
*/
if (dentry->d_lockref.count) {
- list_del_init(&dentry->d_lru);
+ d_lru_isolate(dentry);
spin_unlock(&dentry->d_lock);
return LRU_REMOVED;
}
@@ -925,9 +1000,7 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
return LRU_ROTATE;
}

- dentry->d_flags |= DCACHE_SHRINK_LIST;
- list_move_tail(&dentry->d_lru, freeable);
- this_cpu_dec(nr_dentry_unused);
+ d_lru_shrink_move(dentry, freeable);
spin_unlock(&dentry->d_lock);

return LRU_REMOVED;
@@ -972,9 +1045,7 @@ static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
if (!spin_trylock(&dentry->d_lock))
return LRU_SKIP;

- dentry->d_flags |= DCACHE_SHRINK_LIST;
- list_move_tail(&dentry->d_lru, freeable);
- this_cpu_dec(nr_dentry_unused);
+ d_lru_shrink_move(dentry, freeable);
spin_unlock(&dentry->d_lock);

return LRU_REMOVED;
@@ -1362,9 +1433,13 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
if (dentry->d_lockref.count) {
dentry_lru_del(dentry);
} else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
- dentry_lru_del(dentry);
- list_add_tail(&dentry->d_lru, &data->dispose);
- dentry->d_flags |= DCACHE_SHRINK_LIST;
+ /*
+ * We can't use d_lru_shrink_move() because we
+ * need to get the global LRU lock and do the
+ * RLU accounting.
+ */
+ d_lru_del(dentry);
+ d_shrink_add(dentry, &data->dispose);
data->found++;
ret = D_WALK_NORETRY;
}