Commit ba65dc5e authored by Al Viro's avatar Al Viro

much milder d_walk() race

d_walk() relies upon the tree not getting rearranged under it without
rename_lock being touched.  And we do grab rename_lock around the
places that change the tree topology.  Unfortunately, branch reordering
is just as bad from d_walk() POV and we have two places that do it
without touching rename_lock - one in handling of cursors (for ramfs-style
directories) and another in autofs.  autofs one is a separate story; this
commit deals with the cursors.
	* mark cursor dentries explicitly at allocation time
	* make __dentry_kill() leave ->d_child.next pointing to the next
non-cursor sibling, making sure that it won't be moved around unnoticed
before the parent is relocked on ascend-to-parent path in d_walk().
	* make d_walk() skip cursors explicitly; strictly speaking it's
not necessary (all callbacks we pass to d_walk() are no-ops on cursors),
but it makes analysis easier.
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 1607f09c
......@@ -507,6 +507,44 @@ void d_drop(struct dentry *dentry)
}
EXPORT_SYMBOL(d_drop);
static inline void dentry_unlist(struct dentry *dentry, struct dentry *parent)
{
struct dentry *next;
/*
* Inform d_walk() and shrink_dentry_list() that we are no longer
* attached to the dentry tree
*/
dentry->d_flags |= DCACHE_DENTRY_KILLED;
if (unlikely(list_empty(&dentry->d_child)))
return;
__list_del_entry(&dentry->d_child);
/*
* Cursors can move around the list of children. While we'd been
* a normal list member, it didn't matter - ->d_child.next would've
* been updated. However, from now on it won't be and for the
* things like d_walk() it might end up with a nasty surprise.
* Normally d_walk() doesn't care about cursors moving around -
* ->d_lock on parent prevents that and since a cursor has no children
* of its own, we get through it without ever unlocking the parent.
* There is one exception, though - if we ascend from a child that
* gets killed as soon as we unlock it, the next sibling is found
* using the value left in its ->d_child.next. And if _that_
* pointed to a cursor, and cursor got moved (e.g. by lseek())
* before d_walk() regains parent->d_lock, we'll end up skipping
* everything the cursor had been moved past.
*
* Solution: make sure that the pointer left behind in ->d_child.next
* points to something that won't be moving around. I.e. skip the
* cursors.
*/
while (dentry->d_child.next != &parent->d_subdirs) {
next = list_entry(dentry->d_child.next, struct dentry, d_child);
if (likely(!(next->d_flags & DCACHE_DENTRY_CURSOR)))
break;
dentry->d_child.next = next->d_child.next;
}
}
static void __dentry_kill(struct dentry *dentry)
{
struct dentry *parent = NULL;
......@@ -532,12 +570,7 @@ static void __dentry_kill(struct dentry *dentry)
}
/* if it was on the hash then remove it */
__d_drop(dentry);
__list_del_entry(&dentry->d_child);
/*
* Inform d_walk() that we are no longer attached to the
* dentry tree
*/
dentry->d_flags |= DCACHE_DENTRY_KILLED;
dentry_unlist(dentry, parent);
if (parent)
spin_unlock(&parent->d_lock);
dentry_iput(dentry);
......@@ -1203,6 +1236,9 @@ static void d_walk(struct dentry *parent, void *data,
struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
next = tmp->next;
if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR))
continue;
spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
ret = enter(data, dentry);
......@@ -1651,6 +1687,16 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
}
EXPORT_SYMBOL(d_alloc);
struct dentry *d_alloc_cursor(struct dentry * parent)
{
struct dentry *dentry = __d_alloc(parent->d_sb, NULL);
if (dentry) {
dentry->d_flags |= DCACHE_RCUACCESS | DCACHE_DENTRY_CURSOR;
dentry->d_parent = dget(parent);
}
return dentry;
}
/**
* d_alloc_pseudo - allocate a dentry (for lookup-less filesystems)
* @sb: the superblock
......
......@@ -130,6 +130,7 @@ extern int invalidate_inodes(struct super_block *, bool);
extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
extern int d_set_mounted(struct dentry *dentry);
extern long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc);
extern struct dentry *d_alloc_cursor(struct dentry *);
/*
* read_write.c
......
......@@ -71,9 +71,7 @@ EXPORT_SYMBOL(simple_lookup);
int dcache_dir_open(struct inode *inode, struct file *file)
{
static struct qstr cursor_name = QSTR_INIT(".", 1);
file->private_data = d_alloc(file->f_path.dentry, &cursor_name);
file->private_data = d_alloc_cursor(file->f_path.dentry);
return file->private_data ? 0 : -ENOMEM;
}
......
......@@ -212,6 +212,7 @@ struct dentry_operations {
#define DCACHE_OP_REAL 0x08000000
#define DCACHE_PAR_LOOKUP 0x10000000 /* being looked up (with parent locked shared) */
#define DCACHE_DENTRY_CURSOR 0x20000000
extern seqlock_t rename_lock;
......
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