locks: make locks_mandatory_area check for file-private locks

Allow locks_mandatory_area() to handle file-private locks correctly.
If there is a file-private lock set on an open file and we're doing I/O
via the same, then that should not cause anything to block.

Handle this by first doing a non-blocking FL_ACCESS check for a
file-private lock, and then fall back to checking for a classic POSIX
lock (and possibly blocking).

Note that this approach is subject to the same races that have always
plagued mandatory locking on Linux.

Reported-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/fs/locks.c b/fs/locks.c
index d82c51c4fcd2..13fc7a6d380a 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1199,19 +1199,30 @@ int locks_mandatory_area(int read_write, struct inode *inode,
 {
         struct file_lock fl;
         int error;
+        bool sleep = false;
 
         locks_init_lock(&fl);
-        fl.fl_owner = current->files;
         fl.fl_pid = current->tgid;
         fl.fl_file = filp;
         fl.fl_flags = FL_POSIX | FL_ACCESS;
         if (filp && !(filp->f_flags & O_NONBLOCK))
-                fl.fl_flags |= FL_SLEEP;
+                sleep = true;
         fl.fl_type = (read_write == FLOCK_VERIFY_WRITE) ? F_WRLCK : F_RDLCK;
         fl.fl_start = offset;
         fl.fl_end = offset + count - 1;
 
         for (;;) {
+                if (filp) {
+                        fl.fl_owner = (fl_owner_t)filp;
+                        fl.fl_flags &= ~FL_SLEEP;
+                        error = __posix_lock_file(inode, &fl, NULL);
+                        if (!error)
+                                break;
+                }
+
+                if (sleep)
+                        fl.fl_flags |= FL_SLEEP;
+                fl.fl_owner = current->files;
                 error = __posix_lock_file(inode, &fl, NULL);
                 if (error != FILE_LOCK_DEFERRED)
                         break;