On Mon, Jun 09, 2014 at 12:30:34PM +0900, J. R. Okajima wrote:
>
> Linus Torvalds:
> > So I ended up doing an rc8 because I was a bit worried about some
> > last-minute dcache fixes, but it turns out that nobody seemed to even
> > notice those. We did have other issues during the week, though, so it
> :::
>
> I am afraid there is a problem in dcache. Please read
> http://marc.info/?l=linux-fsdevel&m=140214911608925&w=2
There is a problem, all right, but your fix doesn't really fix it - just
narrows the race window ;-/ I would really like to detach the bugger
as soon as __dentry_kill() removes it from the list of children, but
unfortunately NFS wants it to be still valid in ->d_iput() (BTW,
nfs_can_unlink() is doing something very odd -
parent = dget_parent(dentry);
if (parent == NULL)
goto out_free;
is pointless, since dget_parent() never returns NULL; what was that
check trying to accomplish?)
Your scenario isn't feasible as described, but something similar can
happen with *two* shrinkers racing - dirB might've ended up on one list,
with fileC looked up a bit later, ending up on another list right after
that. So the problem is real; unfortunately, DCACHE_DENTRY_KILLED might
have appeared right after we'd dropped ->d_lock.
So I suspect that the right fix is a bit trickier - in addition to check
on the fast path (i.e. when trylock gets us the lock on parent), we need
to
* get rcu_read_lock() before dropping ->d_lock.
* check if dentry is already doomed right after taking rcu_read_lock();
if not, any value we might see in ->d_parent afterwards will point to object
not freed until we drop rcu_read_lock.
IOW, something like the delta below. Comments?
PS: apologies for being MIA; caught some crap, spent the last week being
very unhappy ;-/ I'll send a pull request tomorrow morning.
diff --git a/fs/dcache.c b/fs/dcache.c
index be2bea8..65ec10f 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -532,10 +532,16 @@ static inline struct dentry *lock_parent(struct dentry
*dentry)
struct dentry *parent = dentry->d_parent;
if (IS_ROOT(dentry))
return NULL;
+ if (unlikely((int)dentry->d_lockref.count < 0))
+ return NULL;
if (likely(spin_trylock(&parent->d_lock)))
return parent;
- spin_unlock(&dentry->d_lock);
rcu_read_lock();
+ if (unlikely((int)dentry->d_lockref.count < 0)) {
+ rcu_read_unlock();
+ return NULL;
+ }
+ spin_unlock(&dentry->d_lock);
again:
parent = ACCESS_ONCE(dentry->d_parent);
spin_lock(&parent->d_lock);
--
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