bug fix: prevent self deadlock with remount code in pivot_root scenarios

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/fs/unionfs/inode.c b/fs/unionfs/inode.c
index fcdd7398191e8d4c94353897323cc3b3869428b6..dfe516eb9818b396a74d132058938853e5f06ae7 100644 (file)
--- a/fs/unionfs/inode.c
+++ b/fs/unionfs/inode.c
@@ -967,7 +967,23 @@ static int unionfs_permission(struct inode *inode, int mask,
        const int is_file = !S_ISDIR(inode->i_mode);
        const int write_mask = (mask & MAY_WRITE) && !(mask & MAY_READ);
 
-       unionfs_read_lock(inode->i_sb);
+       /*
+        * If the same process which is pivot_root'ed on a unionfs, tries to
+        * insert a new branch, then the caller (remount code) already has
+        * the write lock on this rwsem.  It then calls here to check the
+        * permission of a new branch to add.  It could get into a self
+        * deadlock with this attempt to get the read lock (which is crucial
+        * for dynamic branch-management) unless no one else is waiting on
+        * this lock.  Essentially this test tries to figure out if the same
+        * process which also holds a write lock on the rwsem, also tries to
+        * grab a read lock, and then skip trying to grab this "harmless"
+        * read lock; otherwise we DO want to grab the read lock, and block
+        * as needed (dynamic branch management).  (BTW, if there's a better
+        * way to find out who is the lock owner compared to "current", that
+        * should be used instead.)
+        */
+       if (!list_empty(&UNIONFS_SB(inode->i_sb)->rwsem.wait_list))
+               unionfs_read_lock(inode->i_sb);
 
        bstart = ibstart(inode);
        bend = ibend(inode);
@@ -1021,7 +1037,8 @@ static int unionfs_permission(struct inode *inode, int mask,
        }
 
 out:
-       unionfs_read_unlock(inode->i_sb);
+       if (!list_empty(&UNIONFS_SB(inode->i_sb)->rwsem.wait_list))
+               unionfs_read_unlock(inode->i_sb);
        unionfs_check_inode(inode);
        return err;
 }