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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@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
David Howells 2015-03-05 14:09:22 +00:00 committed by Al Viro
parent 525d27b235
commit 4bf46a2726
2 changed files with 42 additions and 26 deletions
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47
fs/dcache.c
View File

@ -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);

21
include/linux/dcache.h
View File

@ -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)