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Message-ID: <20150325144534.17670.31293.stgit@warthog.procyon.org.uk>
Date: Wed, 25 Mar 2015 14:45:34 +0000
From: David Howells <dhowells@...hat.com>
To: viro@....linux.org.uk
Cc: dhowells@...hat.com, linux-unionfs@...r.kernel.org,
linux-kernel@...r.kernel.org, miklos@...redi.hu
Subject: [PATCH 13/15] VFS: Impose ordering on accesses of d_inode and
d_flags
Impose ordering on accesses of d_inode and d_flags to avoid the need to do
this:
if (!dentry->d_inode || d_is_negative(dentry)) {
when this:
if (d_is_negative(dentry)) {
should suffice.
This check is especially problematic if a dentry can have its type field set
to something other than DENTRY_MISS_TYPE when d_inode is NULL (as in
unionmount).
What we really need to do is stick a write barrier between setting d_inode and
setting d_flags and a read barrier between reading d_flags and reading
d_inode.
Signed-off-by: David Howells <dhowells@...hat.com>
---
fs/dcache.c | 47 +++++++++++++++++++++++++++++++++++++++--------
include/linux/dcache.h | 21 +++------------------
2 files changed, 42 insertions(+), 26 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index c71e3732e53b..b9e8b41459a4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -269,6 +269,41 @@ static inline int dname_external(const struct dentry *dentry)
return dentry->d_name.name != dentry->d_iname;
}
+/*
+ * Make sure other CPUs see the inode attached before the type is set.
+ */
+static inline void __d_set_inode_and_type(struct dentry *dentry,
+ struct inode *inode,
+ unsigned type_flags)
+{
+ unsigned flags;
+
+ dentry->d_inode = inode;
+ smp_wmb();
+ flags = READ_ONCE(dentry->d_flags);
+ flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
+ flags |= type_flags;
+ WRITE_ONCE(dentry->d_flags, flags);
+}
+
+/*
+ * Ideally, we want to make sure that other CPUs see the flags cleared before
+ * the inode is detached, but this is really a violation of RCU principles
+ * since the ordering suggests we should always set inode before flags.
+ *
+ * We should instead replace or discard the entire dentry - but that sucks
+ * performancewise on mass deletion/rename.
+ */
+static inline void __d_clear_type_and_inode(struct dentry *dentry)
+{
+ unsigned flags = READ_ONCE(dentry->d_flags);
+
+ flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
+ WRITE_ONCE(dentry->d_flags, flags);
+ smp_wmb();
+ dentry->d_inode = NULL;
+}
+
static void dentry_free(struct dentry *dentry)
{
WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
@@ -311,7 +346,7 @@ static void dentry_iput(struct dentry * dentry)
{
struct inode *inode = dentry->d_inode;
if (inode) {
- dentry->d_inode = NULL;
+ __d_clear_type_and_inode(dentry);
hlist_del_init(&dentry->d_u.d_alias);
spin_unlock(&dentry->d_lock);
spin_unlock(&inode->i_lock);
@@ -335,8 +370,7 @@ static void dentry_unlink_inode(struct dentry * dentry)
__releases(dentry->d_inode->i_lock)
{
struct inode *inode = dentry->d_inode;
- __d_clear_type(dentry);
- dentry->d_inode = NULL;
+ __d_clear_type_and_inode(dentry);
hlist_del_init(&dentry->d_u.d_alias);
dentry_rcuwalk_barrier(dentry);
spin_unlock(&dentry->d_lock);
@@ -1715,11 +1749,9 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
unsigned add_flags = d_flags_for_inode(inode);
spin_lock(&dentry->d_lock);
- dentry->d_flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
- dentry->d_flags |= add_flags;
if (inode)
hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
- dentry->d_inode = inode;
+ __d_set_inode_and_type(dentry, inode, add_flags);
dentry_rcuwalk_barrier(dentry);
spin_unlock(&dentry->d_lock);
fsnotify_d_instantiate(dentry, inode);
@@ -1937,8 +1969,7 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected)
add_flags |= DCACHE_DISCONNECTED;
spin_lock(&tmp->d_lock);
- tmp->d_inode = inode;
- tmp->d_flags |= add_flags;
+ __d_set_inode_and_type(tmp, inode, add_flags);
hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry);
hlist_bl_lock(&tmp->d_sb->s_anon);
hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index e83768ee38fc..df334cbacc6d 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -404,26 +404,11 @@ static inline bool d_mountpoint(const struct dentry *dentry)
/*
* Directory cache entry type accessor functions.
*/
-static inline void __d_set_type(struct dentry *dentry, unsigned type)
-{
- dentry->d_flags = (dentry->d_flags & ~DCACHE_ENTRY_TYPE) | type;
-}
-
-static inline void __d_clear_type(struct dentry *dentry)
-{
- __d_set_type(dentry, DCACHE_MISS_TYPE);
-}
-
-static inline void d_set_type(struct dentry *dentry, unsigned type)
-{
- spin_lock(&dentry->d_lock);
- __d_set_type(dentry, type);
- spin_unlock(&dentry->d_lock);
-}
-
static inline unsigned __d_entry_type(const struct dentry *dentry)
{
- return dentry->d_flags & DCACHE_ENTRY_TYPE;
+ unsigned type = READ_ONCE(dentry->d_flags);
+ smp_rmb();
+ return type & DCACHE_ENTRY_TYPE;
}
static inline bool d_is_miss(const struct dentry *dentry)
--
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