vfs: take f_lock on modifying f_mode after open time

commit 42e49608 upstream.

We'll introduce FMODE_RANDOM which will be runtime modified.  So protect
all runtime modification to f_mode with f_lock to avoid races.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/file_table.c

@@ -420,7 +420,9 @@ void mark_files_ro(struct super_block *sb)
 			continue;
 		if (!(f->f_mode & FMODE_WRITE))
 			continue;
+		spin_lock(&f->f_lock);
 		f->f_mode &= ~FMODE_WRITE;
+		spin_unlock(&f->f_lock);
 		if (file_check_writeable(f) != 0)
 			continue;
 		file_release_write(f);

fs/nfsd/nfs4state.c

@@ -2002,7 +2002,9 @@ nfs4_file_downgrade(struct file *filp, unsigned int share_access)
 {
 	if (share_access & NFS4_SHARE_ACCESS_WRITE) {
 		drop_file_write_access(filp);
+		spin_lock(&filp->f_lock);
 		filp->f_mode = (filp->f_mode | FMODE_READ) & ~FMODE_WRITE;
+		spin_unlock(&filp->f_lock);
 	}
 }