[GFS2] flocks from same process trip kernel BUG at fs/gfs2/glock.c:1118!

This patch adds a new flag to the gfs2_holder structure GL_FLOCK.
It is set on holders of glocks representing flocks. This flag is
checked in add_to_queue() and a process is permitted to queue more
than one holder onto a glock if it is set. This solves the issue
of a process not being able to do multiple flocks on the same file.
Through a single descriptor, a process can now promote and demote
flocks. Through multiple descriptors a process can now queue
multiple flocks on the same file. There's still the problem of
a process deadlocking itself (because gfs2 blocking locks are not
interruptible) by queueing incompatible deadlock.

Signed-off-by: Abhijith Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 6 insertions, 7 deletions

diff --git a/fs/gfs2/ops_file.c b/fs/gfs2/ops_file.c
index 94d76ace0b95..46a9e10ff17b 100644
--- a/fs/gfs2/ops_file.c
+++ b/fs/gfs2/ops_file.c
@@ -571,7 +571,8 @@ static int do_flock(struct file *file, int cmd, struct file_lock *fl)
         int error = 0;
 
         state = (fl->fl_type == F_WRLCK) ? LM_ST_EXCLUSIVE : LM_ST_SHARED;
-        flags = (IS_SETLKW(cmd) ? 0 : LM_FLAG_TRY) | GL_EXACT | GL_NOCACHE;
+        flags = (IS_SETLKW(cmd) ? 0 : LM_FLAG_TRY) | GL_EXACT | GL_NOCACHE 
+                | GL_FLOCK;
 
         mutex_lock(&fp->f_fl_mutex);
 
@@ -579,21 +580,19 @@ static int do_flock(struct file *file, int cmd, struct file_lock *fl)
         if (gl) {
                 if (fl_gh->gh_state == state)
                         goto out;
-                gfs2_glock_hold(gl);
                 flock_lock_file_wait(file,
                                      &(struct file_lock){.fl_type = F_UNLCK});
-                gfs2_glock_dq_uninit(fl_gh);
+                gfs2_glock_dq_wait(fl_gh);
+                gfs2_holder_reinit(state, flags, fl_gh);
         } else {
                 error = gfs2_glock_get(GFS2_SB(&ip->i_inode),
                                       ip->i_no_addr, &gfs2_flock_glops,
                                       CREATE, &gl);
                 if (error)
                         goto out;
+                gfs2_holder_init(gl, state, flags, fl_gh);
+                gfs2_glock_put(gl);
         }
-
-        gfs2_holder_init(gl, state, flags, fl_gh);
-        gfs2_glock_put(gl);
-
         error = gfs2_glock_nq(fl_gh);
         if (error) {
                 gfs2_holder_uninit(fl_gh);