smb: client: fix potential deadlock when releasing mids

All release_mid() callers seem to hold a reference of @mid so there is
no need to call kref_put(&mid->refcount, __release_mid) under
@server->mid_lock spinlock.  If they don't, then an use-after-free bug
would have occurred anyways.

By getting rid of such spinlock also fixes a potential deadlock as
shown below

CPU 0                                CPU 1
------------------------------------------------------------------
cifs_demultiplex_thread()            cifs_debug_data_proc_show()
 release_mid()
  spin_lock(&server->mid_lock);
                                     spin_lock(&cifs_tcp_ses_lock)
				      spin_lock(&server->mid_lock)
  __release_mid()
   smb2_find_smb_tcon()
    spin_lock(&cifs_tcp_ses_lock) *deadlock*

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Paulo Alcantara

fs/smb/client/cifsproto.h

@@ -81,7 +81,7 @@ extern char *cifs_build_path_to_root(struct smb3_fs_context *ctx,
 extern char *build_wildcard_path_from_dentry(struct dentry *direntry);
 char *cifs_build_devname(char *nodename, const char *prepath);
 extern void delete_mid(struct mid_q_entry *mid);
-extern void release_mid(struct mid_q_entry *mid);
+void __release_mid(struct kref *refcount);
 extern void cifs_wake_up_task(struct mid_q_entry *mid);
 extern int cifs_handle_standard(struct TCP_Server_Info *server,
 				struct mid_q_entry *mid);
@@ -740,4 +740,9 @@ static inline bool dfs_src_pathname_equal(const char *s1, const char *s2)
 	return true;
 }
 
+static inline void release_mid(struct mid_q_entry *mid)
+{
+	kref_put(&mid->refcount, __release_mid);
+}
+
 #endif			/* _CIFSPROTO_H */

fs/smb/client/smb2misc.c

@@ -787,7 +787,7 @@ __smb2_handle_cancelled_cmd(struct cifs_tcon *tcon, __u16 cmd, __u64 mid,
 {
 	struct close_cancelled_open *cancelled;
 
-	cancelled = kzalloc(sizeof(*cancelled), GFP_ATOMIC);
+	cancelled = kzalloc(sizeof(*cancelled), GFP_KERNEL);
 	if (!cancelled)
 		return -ENOMEM;
 

fs/smb/client/transport.c

@@ -76,7 +76,7 @@ alloc_mid(const struct smb_hdr *smb_buffer, struct TCP_Server_Info *server)
 	return temp;
 }
 
-static void __release_mid(struct kref *refcount)
+void __release_mid(struct kref *refcount)
 {
 	struct mid_q_entry *midEntry =
 			container_of(refcount, struct mid_q_entry, refcount);
@@ -156,15 +156,6 @@ static void __release_mid(struct kref *refcount)
 	mempool_free(midEntry, cifs_mid_poolp);
 }
 
-void release_mid(struct mid_q_entry *mid)
-{
-	struct TCP_Server_Info *server = mid->server;
-
-	spin_lock(&server->mid_lock);
-	kref_put(&mid->refcount, __release_mid);
-	spin_unlock(&server->mid_lock);
-}
-
 void
 delete_mid(struct mid_q_entry *mid)
 {