[patch] mm: tiny-shmem fix lor, mmap_sem vs i_mutex

[Nick Piggin <npiggin@suse.de>]

On Sun, Sep 14, 2008 at 10:06:31AM +0200, Ingo Molnar wrote:
> 
> * Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > > [ 6460.634452] 
> > > [ 6460.634465] =======================================================
> > > [ 6460.634494] [ INFO: possible circular locking dependency detected ]
> > > [ 6460.634517] 2.6.27-rc6-tip-00290-g8e229c3-dirty #1
> > > [ 6460.634535] -------------------------------------------------------
> > > [ 6460.634555] gdm-simple-gree/4778 is trying to acquire lock:
> > > [ 6460.634574]  (&mm->mmap_sem){----}, at: [<c018fe33>] might_fault+0x36/0x73
> > > [ 6460.634639] 
> > > [ 6460.634645] but task is already holding lock:
> > > [ 6460.634662]  (&dev->ev_mutex){--..}, at: [<c01c7a76>] inotify_read+0xd8/0x16e
> > > [ 6460.634715] 
> > > [ 6460.634721] which lock already depends on the new lock.
> > 
> > Yes, there's a thread in my intray called "inotify_read's ev_mutex vs 
> > do_page_fault's mmap_sem...".  It's a bit flakey-looking, but there's 
> > a patch in there.
> 
> ah, thx. I picked up the patch into tip/out-of-tree. (see below for a 
> tided up changelog) Please queue it up as v2.6.27 material. (i'll report 
> it if anything breaks due to the patch)
> 
> 	Ingo
> 
> --------------->
> >From 1eb0a42e4eb3283521ee1de99adbf567874b622f Mon Sep 17 00:00:00 2001
> From: Nick Piggin <nickpiggin@yahoo.com.au>
> Date: Thu, 11 Sep 2008 06:12:51 +1000
> Subject: [PATCH] mm: fix locking, inotify_read's ev_mutex vs do_page_fault's mmap_sem...

I would call this "fs: fix inotify locking....", it's not really an mm bug
if another subsystem misuses mm's APIs. But that's a nitpick.

Here is the other patch I did too.


tiny-shmem calls do_truncate in shmem_file_setup. do_truncate takes i_mutex,
and shmem_file_setup is called with mmap_sem held. However i_mutex nests
outside mmap_sem.

Copy the code in shmem.c to avoid this problem.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/tiny-shmem.c
===================================================================
--- linux-2.6.orig/mm/tiny-shmem.c
+++ linux-2.6/mm/tiny-shmem.c
@@ -65,31 +65,25 @@ struct file *shmem_file_setup(char *name
 	if (!dentry)
 		goto put_memory;
 
+        error = -ENFILE;
+        file = get_empty_filp();
+        if (!file)
+                goto put_dentry;
+
 	error = -ENOSPC;
 	inode = ramfs_get_inode(root->d_sb, S_IFREG | S_IRWXUGO, 0);
 	if (!inode)
-		goto put_dentry;
-
-	d_instantiate(dentry, inode);
-	error = -ENFILE;
-	file = alloc_file(shm_mnt, dentry, FMODE_WRITE | FMODE_READ,
-			&ramfs_file_operations);
-	if (!file)
-		goto put_dentry;
-
-	inode->i_nlink = 0;	/* It is unlinked */
-
-	/* notify everyone as to the change of file size */
-	error = do_truncate(dentry, size, 0, file);
-	if (error < 0)
 		goto close_file;
 
+        d_instantiate(dentry, inode);
+        inode->i_size = size;
+        inode->i_nlink = 0;     /* It is unlinked */
+        init_file(file, shm_mnt, dentry, FMODE_WRITE | FMODE_READ,
+                        &ramfs_file_operations);
 	return file;
 
 close_file:
 	put_filp(file);
-	return ERR_PTR(error);
-
 put_dentry:
 	dput(dentry);
 put_memory: