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vfs: fix dentry RCU to refcounting possibly sleeping dput() 

This is the fix that the last two commits indirectly led up to - making
sure that we don't call dput() in a bad context on the dentries we've
looked up in RCU mode after the sequence count validation fails.

This basically expands d_rcu_to_refcount() into the callers, and then
fixes the callers to delay the dput() in the failure case until _after_
we've dropped all locks and are no longer in an RCU-locked region.

The case of 'complete_walk()' was trivial, since its failure case did
the unlock_rcu_walk() directly after the call to d_rcu_to_refcount(),
and as such that is just a pure expansion of the function with a trivial
movement of the resulting dput() to after 'unlock_rcu_walk()'.

In contrast, the unlazy_walk() case was much more complicated, because
not only does convert two different dentries from RCU to be reference
counted, but it used to not call unlock_rcu_walk() at all, and instead
just returned an error and let the caller clean everything up in
"terminate_walk()".

Happily, one of the dentries in question (called "parent" inside
unlazy_walk()) is the dentry of "nd->path", which terminate_walk() wants
a refcount to anyway for the non-RCU case.

So what the new and improved unlazy_walk() does is to first turn that
dentry into a refcounted one, and once that is set up, the error cases
can continue to use the terminate_walk() helper for cleanup, but for the
non-RCU case.  Which makes it possible to drop out of RCU mode if we
actually hit the sequence number failure case.

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2013-09-08 18:13:49 -07:00
parent 0d98439ea3
commit e5c832d555
1 changed files with 49 additions and 53 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

102
fs/namei.c
View File

@ -494,37 +494,6 @@ static inline void unlock_rcu_walk(void)
br_read_unlock(&vfsmount_lock);
}
/*
* When we move over from the RCU domain to properly refcounted
* long-lived dentries, we need to check the sequence numbers
* we got before lookup very carefully.
*
* We cannot blindly increment a dentry refcount - even if it
* is not locked - if it is zero, because it may have gone
* through the final d_kill() logic already.
*
* So for a zero refcount, we need to get the spinlock (which is
* safe even for a dead dentry because the de-allocation is
* RCU-delayed), and check the sequence count under the lock.
*
* Once we have checked the sequence count, we know it is live,
* and since we hold the spinlock it cannot die from under us.
*
* In contrast, if the reference count wasn't zero, we can just
* increment the lockref without having to take the spinlock.
* Even if the sequence number ends up being stale, we haven't
* gone through the final dput() and killed the dentry yet.
*/
static inline int d_rcu_to_refcount(struct dentry *dentry, seqcount_t *validate, unsigned seq)
{
if (likely(lockref_get_not_dead(&dentry->d_lockref))) {
if (!read_seqcount_retry(validate, seq))
return 0;
dput(dentry);
}
return -ECHILD;
}
/**
* unlazy_walk - try to switch to ref-walk mode.
* @nd: nameidata pathwalk data
@ -539,16 +508,29 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
{
struct fs_struct *fs = current->fs;
struct dentry *parent = nd->path.dentry;
int want_root = 0;
BUG_ON(!(nd->flags & LOOKUP_RCU));
if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
want_root = 1;
spin_lock(&fs->lock);
if (nd->root.mnt != fs->root.mnt ||
nd->root.dentry != fs->root.dentry)
goto err_root;
}
/*
* Get a reference to the parent first: we're
* going to make "path_put(nd->path)" valid in
* non-RCU context for "terminate_walk()".
*
* If this doesn't work, return immediately with
* RCU walking still active (and then we will do
* the RCU walk cleanup in terminate_walk()).
*/
if (!lockref_get_not_dead(&parent->d_lockref))
return -ECHILD;
/*
* After the mntget(), we terminate_walk() will do
* the right thing for non-RCU mode, and all our
* subsequent exit cases should unlock_rcu_walk()
* before returning.
*/
mntget(nd->path.mnt);
nd->flags &= ~LOOKUP_RCU;
/*
* For a negative lookup, the lookup sequence point is the parents
@ -562,30 +544,39 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
* be valid if the child sequence number is still valid.
*/
if (!dentry) {
if (d_rcu_to_refcount(parent, &parent->d_seq, nd->seq) < 0)
goto err_root;
if (read_seqcount_retry(&parent->d_seq, nd->seq))
goto out;
BUG_ON(nd->inode != parent->d_inode);
} else {
if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0)
goto err_root;
if (d_rcu_to_refcount(parent, &dentry->d_seq, nd->seq) < 0)
goto err_parent;
if (!lockref_get_not_dead(&dentry->d_lockref))
goto out;
if (read_seqcount_retry(&dentry->d_seq, nd->seq))
goto drop_dentry;
}
if (want_root) {
/*
* Sequence counts matched. Now make sure that the root is
* still valid and get it if required.
*/
if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
spin_lock(&fs->lock);
if (nd->root.mnt != fs->root.mnt || nd->root.dentry != fs->root.dentry)
goto unlock_and_drop_dentry;
path_get(&nd->root);
spin_unlock(&fs->lock);
}
mntget(nd->path.mnt);
unlock_rcu_walk();
nd->flags &= ~LOOKUP_RCU;
return 0;
err_parent:
unlock_and_drop_dentry:
spin_unlock(&fs->lock);
drop_dentry:
unlock_rcu_walk();
dput(dentry);
err_root:
if (want_root)
spin_unlock(&fs->lock);
return -ECHILD;
out:
unlock_rcu_walk();
return -ECHILD;
}
@ -614,10 +605,15 @@ static int complete_walk(struct nameidata *nd)
if (!(nd->flags & LOOKUP_ROOT))
nd->root.mnt = NULL;
if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0) {
if (unlikely(!lockref_get_not_dead(&dentry->d_lockref))) {
unlock_rcu_walk();
return -ECHILD;
}
if (read_seqcount_retry(&dentry->d_seq, nd->seq)) {
unlock_rcu_walk();
dput(dentry);
return -ECHILD;
}
mntget(nd->path.mnt);
unlock_rcu_walk();
}