From e7d6ef9790bc281f5c29d0132b68031248523fe8 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Mon, 20 Jun 2016 01:35:59 -0400 Subject: [PATCH] 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" Wearing-brown-paperbag: Al Viro Signed-off-by: Al Viro --- fs/dcache.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index b7eddfd35aa5..d6847d7b123d 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2503,7 +2503,6 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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;