CIFS: Fix a deadlock when a file is reopened

If we request reading or writing on a file that needs to be
reopened, it causes the deadlock: we are already holding rw
semaphore for reading and then we try to acquire it for writing
in cifs_relock_file. Fix this by acquiring the semaphore for
reading in cifs_relock_file due to we don't make any changes in
locks and don't need a write access.

CC: <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

1 files changed, 4 insertions, 5 deletions

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 1dc9dea2ae70..1e57f36ea1b2 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -561,11 +561,10 @@ cifs_relock_file(struct cifsFileInfo *cfile)
         struct cifs_tcon *tcon = tlink_tcon(cfile->tlink);
         int rc = 0;
 
-        /* we are going to update can_cache_brlcks here - need a write access */
-        down_write(&cinode->lock_sem);
+        down_read(&cinode->lock_sem);
         if (cinode->can_cache_brlcks) {
-                /* can cache locks - no need to push them */
-                up_write(&cinode->lock_sem);
+                /* can cache locks - no need to relock */
+                up_read(&cinode->lock_sem);
                 return rc;
         }
 
@@ -576,7 +575,7 @@ cifs_relock_file(struct cifsFileInfo *cfile)
         else
                 rc = tcon->ses->server->ops->push_mand_locks(cfile);
 
-        up_write(&cinode->lock_sem);
+        up_read(&cinode->lock_sem);
         return rc;
 }