From 3b3f09f48ba78c0634e929849860a6447d057eed Mon Sep 17 00:00:00 2001 From: Al Viro Date: Fri, 23 Feb 2018 21:54:18 -0500 Subject: get rid of trylock loop in locking dentries on shrink list In case of trylock failure don't re-add to the list - drop the locks and carefully get them in the right order. For shrink_dentry_list(), somebody having grabbed a reference to dentry means that we can kick it off-list, so if we find dentry being modified under us we don't need to play silly buggers with retries anyway - off the list it is. The locking logics taken out into a helper of its own; lock_parent() is no longer used for dentries that can be killed under us. [fix from Eric Biggers folded] Signed-off-by: Al Viro --- fs/dcache.c | 104 +++++++++++++++++++++++++++++++++++++++--------------------- 1 file changed, 67 insertions(+), 37 deletions(-) (limited to 'fs/dcache.c') diff --git a/fs/dcache.c b/fs/dcache.c index 1684b6b262de..af8501489af5 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -974,56 +974,86 @@ restart: } EXPORT_SYMBOL(d_prune_aliases); -static void shrink_dentry_list(struct list_head *list) +/* + * Lock a dentry from shrink list. + * Note that dentry is *not* protected from concurrent dentry_kill(), + * d_delete(), etc. It is protected from freeing (by the fact of + * being on a shrink list), but everything else is fair game. + * Return false if dentry has been disrupted or grabbed, leaving + * the caller to kick it off-list. Otherwise, return true and have + * that dentry's inode and parent both locked. + */ +static bool shrink_lock_dentry(struct dentry *dentry) { - struct dentry *dentry, *parent; + struct inode *inode; + struct dentry *parent; - while (!list_empty(list)) { - struct inode *inode; - dentry = list_entry(list->prev, struct dentry, d_lru); + if (dentry->d_lockref.count) + return false; + + inode = dentry->d_inode; + if (inode && unlikely(!spin_trylock(&inode->i_lock))) { + rcu_read_lock(); /* to protect inode */ + spin_unlock(&dentry->d_lock); + spin_lock(&inode->i_lock); spin_lock(&dentry->d_lock); - parent = lock_parent(dentry); + if (unlikely(dentry->d_lockref.count)) + goto out; + /* changed inode means that somebody had grabbed it */ + if (unlikely(inode != dentry->d_inode)) + goto out; + rcu_read_unlock(); + } - /* - * The dispose list is isolated and dentries are not accounted - * to the LRU here, so we can simply remove it from the list - * here regardless of whether it is referenced or not. - */ - d_shrink_del(dentry); + parent = dentry->d_parent; + if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock))) + return true; - /* - * We found an inuse dentry which was not removed from - * the LRU because of laziness during lookup. Do not free it. - */ - if (dentry->d_lockref.count > 0) { - spin_unlock(&dentry->d_lock); - if (parent) - spin_unlock(&parent->d_lock); - continue; - } + rcu_read_lock(); /* to protect parent */ + spin_unlock(&dentry->d_lock); + parent = READ_ONCE(dentry->d_parent); + spin_lock(&parent->d_lock); + if (unlikely(parent != dentry->d_parent)) { + spin_unlock(&parent->d_lock); + spin_lock(&dentry->d_lock); + goto out; + } + spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED); + if (likely(!dentry->d_lockref.count)) { + rcu_read_unlock(); + return true; + } + spin_unlock(&parent->d_lock); +out: + if (inode) + spin_unlock(&inode->i_lock); + rcu_read_unlock(); + return false; +} +static void shrink_dentry_list(struct list_head *list) +{ + while (!list_empty(list)) { + struct dentry *dentry, *parent; + struct inode *inode; - if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) { - bool can_free = dentry->d_flags & DCACHE_MAY_FREE; + dentry = list_entry(list->prev, struct dentry, d_lru); + spin_lock(&dentry->d_lock); + if (!shrink_lock_dentry(dentry)) { + bool can_free = false; + d_shrink_del(dentry); + if (dentry->d_lockref.count < 0) + can_free = dentry->d_flags & DCACHE_MAY_FREE; spin_unlock(&dentry->d_lock); - if (parent) - spin_unlock(&parent->d_lock); if (can_free) dentry_free(dentry); continue; } - - inode = dentry->d_inode; - if (inode && unlikely(!spin_trylock(&inode->i_lock))) { - d_shrink_add(dentry, list); - spin_unlock(&dentry->d_lock); - if (parent) - spin_unlock(&parent->d_lock); - continue; - } - + d_shrink_del(dentry); + parent = dentry->d_parent; __dentry_kill(dentry); - + if (parent == dentry) + continue; /* * We need to prune ancestors too. This is necessary to prevent * quadratic behavior of shrink_dcache_parent(), but is also -- cgit v1.2.3-59-g8ed1b