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fix dget_parent() fastpath race

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[ 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>
This commit is contained in:
Al Viro 2019-10-31 01:43:31 -04:00 committed by Greg Kroah-Hartman
parent 797f6c19ab
commit 424388f0c5
1 changed files with 3 additions and 1 deletions
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4
fs/dcache.c
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@ -903,17 +903,19 @@ struct dentry *dget_parent(struct dentry *dentry)
{ {
int gotref; int gotref;
struct dentry *ret; struct dentry *ret;
unsigned seq;
/* /*
* Do optimistic parent lookup without any * Do optimistic parent lookup without any
* locking. * locking.
*/ */
rcu_read_lock(); rcu_read_lock();
seq = raw_seqcount_begin(&dentry->d_seq);
ret = READ_ONCE(dentry->d_parent); ret = READ_ONCE(dentry->d_parent);
gotref = lockref_get_not_zero(&ret->d_lockref); gotref = lockref_get_not_zero(&ret->d_lockref);
rcu_read_unlock(); rcu_read_unlock();
if (likely(gotref)) { if (likely(gotref)) {
if (likely(ret == READ_ONCE(dentry->d_parent))) if (!read_seqcount_retry(&dentry->d_seq, seq))
return ret; return ret;
dput(ret); dput(ret);
} }