staging/lustre/llite: Close atomic_open race with several openers

Right now, if it's an open of a negative dentry, a race is possible
with several openers who all try to instantiate/rehash the same
dentry and would hit a BUG_ON in d_add.
But in fact if we got a negative dentry in atomic_open, that means
we just revalidated it so no point in talking to MDS at all,
just return ENOENT and make the race go away completely.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Cc: stable <stable@vger.kernel.org> # 4.7+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 24 insertions, 19 deletions

diff --git a/drivers/staging/lustre/lustre/llite/namei.c b/drivers/staging/lustre/lustre/llite/namei.c
index 3664bfd0178b..2c4dc69731e8 100644
--- a/drivers/staging/lustre/lustre/llite/namei.c
+++ b/drivers/staging/lustre/lustre/llite/namei.c
@@ -388,6 +388,7 @@ static int ll_lookup_it_finish(struct ptlrpc_request *request,
         struct inode *inode = NULL;
         __u64 bits = 0;
         int rc = 0;
+        struct dentry *alias;
 
         /* NB 1 request reference will be taken away by ll_intent_lock()
          * when I return
@@ -412,26 +413,12 @@ static int ll_lookup_it_finish(struct ptlrpc_request *request,
                  */
         }
 
-        /* Only hash *de if it is unhashed (new dentry).
-         * Atoimc_open may passing hashed dentries for open.
-         */
-        if (d_unhashed(*de)) {
-                struct dentry *alias;
-
-                alias = ll_splice_alias(inode, *de);
-                if (IS_ERR(alias)) {
-                        rc = PTR_ERR(alias);
-                        goto out;
-                }
-                *de = alias;
-        } else if (!it_disposition(it, DISP_LOOKUP_NEG)  &&
-                   !it_disposition(it, DISP_OPEN_CREATE)) {
-                /* With DISP_OPEN_CREATE dentry will be
-                 * instantiated in ll_create_it.
-                 */
-                LASSERT(!d_inode(*de));
-                d_instantiate(*de, inode);
+        alias = ll_splice_alias(inode, *de);
+        if (IS_ERR(alias)) {
+                rc = PTR_ERR(alias);
+                goto out;
         }
+        *de = alias;
 
         if (!it_disposition(it, DISP_LOOKUP_NEG)) {
                 /* we have lookup look - unhide dentry */
@@ -587,6 +574,24 @@ static int ll_atomic_open(struct inode *dir, struct dentry *dentry,
                dentry, PFID(ll_inode2fid(dir)), dir, file, open_flags, mode,
                *opened);
 
+        /* Only negative dentries enter here */
+        LASSERT(!d_inode(dentry));
+
+        if (!d_in_lookup(dentry)) {
+                /* A valid negative dentry that just passed revalidation,
+                 * there's little point to try and open it server-side,
+                 * even though there's a minuscle chance it might succeed.
+                 * Either way it's a valid race to just return -ENOENT here.
+                 */
+                if (!(open_flags & O_CREAT))
+                        return -ENOENT;
+
+                /* Otherwise we just unhash it to be rehashed afresh via
+                 * lookup if necessary
+                 */
+                d_drop(dentry);
+        }
+
         it = kzalloc(sizeof(*it), GFP_NOFS);
         if (!it)
                 return -ENOMEM;