From: Al Viro viro@zeniv.linux.org.uk
[ Upstream commit e84009336711d2bba885fc9cea66348ddfce3758 ]
We are overoptimistic about taking the fast path there; seeing the same value in ->d_parent after having grabbed a reference to that parent does *not* mean that it has remained our parent all along.
That wouldn't be a big deal (in the end it is our parent and we have grabbed the reference we are about to return), but... the situation with barriers is messed up.
We might have hit the following sequence:
d is a dentry of /tmp/a/b CPU1: CPU2: parent = d->d_parent (i.e. dentry of /tmp/a) rename /tmp/a/b to /tmp/b rmdir /tmp/a, making its dentry negative grab reference to parent, end up with cached parent->d_inode (NULL) mkdir /tmp/a, rename /tmp/b to /tmp/a/b recheck d->d_parent, which is back to original decide that everything's fine and return the reference we'd got.
The trouble is, caller (on CPU1) will observe dget_parent() returning an apparently negative dentry. It actually is positive, but CPU1 has stale ->d_inode cached.
Use d->d_seq to see if it has been moved instead of rechecking ->d_parent. NOTE: we are *NOT* going to retry on any kind of ->d_seq mismatch; we just go into the slow path in such case. We don't wait for ->d_seq to become even either - again, if we are racing with renames, we can bloody well go to slow path anyway.
Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Sasha Levin sashal@kernel.org --- fs/dcache.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/fs/dcache.c b/fs/dcache.c index 6e0022326afe3..20370a0997bf9 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -864,17 +864,19 @@ struct dentry *dget_parent(struct dentry *dentry) { int gotref; struct dentry *ret; + unsigned seq;
/* * Do optimistic parent lookup without any * locking. */ rcu_read_lock(); + seq = raw_seqcount_begin(&dentry->d_seq); ret = READ_ONCE(dentry->d_parent); gotref = lockref_get_not_zero(&ret->d_lockref); rcu_read_unlock(); if (likely(gotref)) { - if (likely(ret == READ_ONCE(dentry->d_parent))) + if (!read_seqcount_retry(&dentry->d_seq, seq)) return ret; dput(ret); }