fix idiotic braino in d_alloc_parallel()

Check for d_unhashed() while searching in in-lookup hash was absolutely
wrong.  Worse, it masked a deadlock on dget() done under bitlock that
nests inside ->d_lock.  Thanks to J. R. Okajima for spotting it.

Spotted-by: "J. R. Okajima" <hooanon05g@gmail.com>
Wearing-brown-paperbag: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index b7eddfd..d6847d7 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2503,7 +2503,6 @@ retry:
                rcu_read_unlock();
                goto retry;
        }
-       rcu_read_unlock();
        /*
         * No changes for the parent since the beginning of d_lookup().
         * Since all removals from the chain happen with hlist_bl_lock(),
@@ -2516,8 +2515,6 @@ retry:
                        continue;
                if (dentry->d_parent != parent)
                        continue;
-               if (d_unhashed(dentry))
-                       continue;
                if (parent->d_flags & DCACHE_OP_COMPARE) {
                        int tlen = dentry->d_name.len;
                        const char *tname = dentry->d_name.name;
@@ -2529,9 +2526,18 @@ retry:
                        if (dentry_cmp(dentry, str, len))
                                continue;
                }
-               dget(dentry);
                hlist_bl_unlock(b);
-               /* somebody is doing lookup for it right now; wait for it */
+               /* now we can try to grab a reference */
+               if (!lockref_get_not_dead(&dentry->d_lockref)) {
+                       rcu_read_unlock();
+                       goto retry;
+               }
+
+               rcu_read_unlock();
+               /*
+                * somebody is likely to be still doing lookup for it;
+                * wait for them to finish
+                */
                spin_lock(&dentry->d_lock);
                d_wait_lookup(dentry);
                /*
@@ -2562,6 +2568,7 @@ retry:
                dput(new);
                return dentry;
        }
+       rcu_read_unlock();
        /* we can't take ->d_lock here; it's OK, though. */
        new->d_flags |= DCACHE_PAR_LOOKUP;
        new->d_wait = wq;