3 files changed, 73 insertions, 26 deletions

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index bc3549c95574..65872340e301 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -53,6 +53,15 @@ const struct file_operations *debugfs_real_fops(const struct file *filp)
 {
         struct debugfs_fsdata *fsd = F_DENTRY(filp)->d_fsdata;
 
+        if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT) {
+                /*
+                 * Urgh, we've been called w/o a protecting
+                 * debugfs_file_get().
+                 */
+                WARN_ON(1);
+                return NULL;
+        }
+
         return fsd->real_fops;
 }
 EXPORT_SYMBOL_GPL(debugfs_real_fops);
@@ -74,9 +83,35 @@ EXPORT_SYMBOL_GPL(debugfs_real_fops);
  */
 int debugfs_file_get(struct dentry *dentry)
 {
-        struct debugfs_fsdata *fsd = dentry->d_fsdata;
+        struct debugfs_fsdata *fsd;
+        void *d_fsd;
+
+        d_fsd = READ_ONCE(dentry->d_fsdata);
+        if (!((unsigned long)d_fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)) {
+                fsd = d_fsd;
+        } else {
+                fsd = kmalloc(sizeof(*fsd), GFP_KERNEL);
+                if (!fsd)
+                        return -ENOMEM;
+
+                fsd->real_fops = (void *)((unsigned long)d_fsd &
+                                        ~DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
+                refcount_set(&fsd->active_users, 1);
+                init_completion(&fsd->active_users_drained);
+                if (cmpxchg(&dentry->d_fsdata, d_fsd, fsd) != d_fsd) {
+                        kfree(fsd);
+                        fsd = READ_ONCE(dentry->d_fsdata);
+                }
+        }
 
-        /* Avoid starvation of removers. */
+        /*
+         * In case of a successful cmpxchg() above, this check is
+         * strictly necessary and must follow it, see the comment in
+         * __debugfs_remove_file().
+         * OTOH, if the cmpxchg() hasn't been executed or wasn't
+         * successful, this serves the purpose of not starving
+         * removers.
+         */
         if (d_unlinked(dentry))
                 return -EIO;
 
@@ -98,7 +133,7 @@ EXPORT_SYMBOL_GPL(debugfs_file_get);
  */
 void debugfs_file_put(struct dentry *dentry)
 {
-        struct debugfs_fsdata *fsd = dentry->d_fsdata;
+        struct debugfs_fsdata *fsd = READ_ONCE(dentry->d_fsdata);
 
         if (refcount_dec_and_test(&fsd->active_users))
                 complete(&fsd->active_users_drained);
@@ -109,10 +144,11 @@ static int open_proxy_open(struct inode *inode, struct file *filp)
 {
         struct dentry *dentry = F_DENTRY(filp);
         const struct file_operations *real_fops = NULL;
-        int r = 0;
+        int r;
 
-        if (debugfs_file_get(dentry))
-                return -ENOENT;
+        r = debugfs_file_get(dentry);
+        if (r)
+                return r == -EIO ? -ENOENT : r;
 
         real_fops = debugfs_real_fops(filp);
         real_fops = fops_get(real_fops);
@@ -233,10 +269,11 @@ static int full_proxy_open(struct inode *inode, struct file *filp)
         struct dentry *dentry = F_DENTRY(filp);
         const struct file_operations *real_fops = NULL;
         struct file_operations *proxy_fops = NULL;
-        int r = 0;
+        int r;
 
-        if (debugfs_file_get(dentry))
-                return -ENOENT;
+        r = debugfs_file_get(dentry);
+        if (r)
+                return r == -EIO ? -ENOENT : r;
 
         real_fops = debugfs_real_fops(filp);
         real_fops = fops_get(real_fops);
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index f587aded46b5..9dca4da059b3 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -184,7 +184,10 @@ static const struct super_operations debugfs_super_operations = {
 
 static void debugfs_release_dentry(struct dentry *dentry)
 {
-        kfree(dentry->d_fsdata);
+        void *fsd = dentry->d_fsdata;
+
+        if (!((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT))
+                kfree(dentry->d_fsdata);
 }
 
 static struct vfsmount *debugfs_automount(struct path *path)
@@ -344,35 +347,25 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 {
         struct dentry *dentry;
         struct inode *inode;
-        struct debugfs_fsdata *fsd;
-
-        fsd = kmalloc(sizeof(*fsd), GFP_KERNEL);
-        if (!fsd)
-                return NULL;
 
         if (!(mode & S_IFMT))
                 mode |= S_IFREG;
         BUG_ON(!S_ISREG(mode));
         dentry = start_creating(name, parent);
 
-        if (IS_ERR(dentry)) {
-                kfree(fsd);
+        if (IS_ERR(dentry))
                 return NULL;
-        }
 
         inode = debugfs_get_inode(dentry->d_sb);
-        if (unlikely(!inode)) {
-                kfree(fsd);
+        if (unlikely(!inode))
                 return failed_creating(dentry);
-        }
 
         inode->i_mode = mode;
         inode->i_private = data;
 
         inode->i_fop = proxy_fops;
-        fsd->real_fops = real_fops;
-        refcount_set(&fsd->active_users, 1);
-        dentry->d_fsdata = fsd;
+        dentry->d_fsdata = (void *)((unsigned long)real_fops |
+                                DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
 
         d_instantiate(dentry, inode);
         fsnotify_create(d_inode(dentry->d_parent), dentry);
@@ -635,8 +628,17 @@ static void __debugfs_remove_file(struct dentry *dentry, struct dentry *parent)
 
         simple_unlink(d_inode(parent), dentry);
         d_delete(dentry);
-        fsd = dentry->d_fsdata;
-        init_completion(&fsd->active_users_drained);
+
+        /*
+         * Paired with the closing smp_mb() implied by a successful
+         * cmpxchg() in debugfs_file_get(): either
+         * debugfs_file_get() must see a dead dentry or we must see a
+         * debugfs_fsdata instance at ->d_fsdata here (or both).
+         */
+        smp_mb();
+        fsd = READ_ONCE(dentry->d_fsdata);
+        if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
+                return;
         if (!refcount_dec_and_test(&fsd->active_users))
                 wait_for_completion(&fsd->active_users_drained);
 }
diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index 0eea99432840..cb1e8139c398 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -25,4 +25,12 @@ struct debugfs_fsdata {
         struct completion active_users_drained;
 };
 
+/*
+ * A dentry's ->d_fsdata either points to the real fops or to a
+ * dynamically allocated debugfs_fsdata instance.
+ * In order to distinguish between these two cases, a real fops
+ * pointer gets its lowest bit set.
+ */
+#define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0)
+
 #endif /* _DEBUGFS_INTERNAL_H_ */