Commit 4bf46a27 authored by David Howells's avatar David Howells Committed by Al Viro
Browse files

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: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 525d27b2

fs/dcache.c

+39 −8
Original line number Diff line number Diff line
@@ -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);

include/linux/dcache.h

+3 −18
Original line number Diff line number Diff line
@@ -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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