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new helper: d_find_alias_rcu()

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similar to d_find_alias(inode), except that
	* the caller must be holding rcu_read_lock()
	* inode must not be freed until matching rcu_read_unlock()
	* result is *NOT* pinned and can only be dereferenced until
the matching rcu_read_unlock().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2021-01-05 14:13:52 -05:00
parent d36a1dd9f7
commit bca585d24a
2 changed files with 27 additions and 0 deletions
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25
fs/dcache.c
View file

@ -1042,6 +1042,31 @@ struct dentry *d_find_alias(struct inode *inode)
} }
EXPORT_SYMBOL(d_find_alias); EXPORT_SYMBOL(d_find_alias);
/*
* Caller MUST be holding rcu_read_lock() and be guaranteed
* that inode won't get freed until rcu_read_unlock().
*/
struct dentry *d_find_alias_rcu(struct inode *inode)
{
struct hlist_head *l = &inode->i_dentry;
struct dentry *de = NULL;
spin_lock(&inode->i_lock);
// ->i_dentry and ->i_rcu are colocated, but the latter won't be
// used without having I_FREEING set, which means no aliases left
if (likely(!(inode->i_state & I_FREEING) && !hlist_empty(l))) {
if (S_ISDIR(inode->i_mode)) {
de = hlist_entry(l->first, struct dentry, d_u.d_alias);
} else {
hlist_for_each_entry(de, l, d_u.d_alias)
if (!d_unhashed(de))
break;
}
}
spin_unlock(&inode->i_lock);
return de;
}
/* /*
* Try to kill dentries associated with this inode. * Try to kill dentries associated with this inode.
* WARNING: you must own a reference to inode. * WARNING: you must own a reference to inode.

2
include/linux/dcache.h
View file

@ -262,6 +262,8 @@ extern void d_tmpfile(struct dentry *, struct inode *);
extern struct dentry *d_find_alias(struct inode *); extern struct dentry *d_find_alias(struct inode *);
extern void d_prune_aliases(struct inode *); extern void d_prune_aliases(struct inode *);
extern struct dentry *d_find_alias_rcu(struct inode *);
/* test whether we have any submounts in a subdir tree */ /* test whether we have any submounts in a subdir tree */
extern int path_has_submounts(const struct path *); extern int path_has_submounts(const struct path *);