vfs-scale, chroot

[J. R. Okajima]

Hello Nick,

I've got a crash around d_lock.

# mount -t nfs host:/dir /nfs
# chroot /nfs

BUG: spinlock recursion on CPU#1, chroot/2524
 lock: ffff88001d106880, .magic: dead4ead, .owner: chroot/2524, .owner_cpu: 1
Call Trace:
 [<ffffffff8128ca52>] ? spin_bug+0xa2/0xf0
 [<ffffffff8128cd53>] ? do_raw_spin_lock+0x193/0x1b0
 [<ffffffff814eaa5e>] ? _raw_spin_lock_nested+0x4e/0x60
 [<ffffffff81137aef>] ? nameidata_dentry_drop_rcu+0xcf/0x1b0
 [<ffffffff814eaab3>] ? _raw_spin_lock+0x43/0x50
 [<ffffffff81137aef>] ? nameidata_dentry_drop_rcu+0xcf/0x1b0
 [<ffffffff81137c1b>] ? d_revalidate+0x4b/0x70
 [<ffffffff81139a75>] ? link_path_walk+0x655/0x1210
 [<ffffffff81138702>] ? path_init_rcu+0x1c2/0x370
 [<ffffffff811387e5>] ? path_init_rcu+0x2a5/0x370
 [<ffffffff81138702>] ? path_init_rcu+0x1c2/0x370
 [<ffffffff811059b3>] ? might_fault+0x53/0xb0
 [<ffffffff8113a9fe>] ? do_path_lookup+0x8e/0x1d0
 [<ffffffff8113b936>] ? user_path_at+0xa6/0xe0
 [<ffffffff8114c167>] ? vfsmount_lock_local_unlock+0x77/0x90
 [<ffffffff814ebab9>] ? retint_swapgs+0x13/0x1b
 [<ffffffff81090b15>] ? trace_hardirqs_on_caller+0x145/0x190
 [<ffffffff8112841e>] ? sys_chdir+0x2e/0x90
 [<ffffffff8100bfd2>] ? system_call_fastpath+0x16/0x1b

It looks like nameidata_dentry_drop_rcu() is trying spin_lock() twice
for the same dentry when parent == dentry.

- NFS ->d_revalidate() returns -ECHILD for LOOKUP_RCU
- VFS d_revalidate() will try ->d_revalidate() again after dropping
  LOOKUP_RCU by nameidata_dentry_drop_rcu().
- nameidata_dentry_drop_rcu() calls
	spin_lock(&parent->d_lock);
	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
- it may happen on all fs which specifies FS_REVAL_DOT

If we have a function like below, it may be useful.
But are there so many cases like this problem?
If it is not so many, then the fix will be adding several
"if (!IS_ROOT(dentry)" into nameidata_dentry_drop_rcu(), I think.

int d_lock_parent_child(parent, child)
{
	err = Success;
	spin_lock(&parent->d_lock);
	if (!IS_ROOT(dentry)) {
		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
		if (unlikely(parent != dentry->d_parent)) {
			spin_unlock(&parent->d_lock);
			err = Error_Unmatch;
		}
	} else
		err = Success_Root;
	return err;
}

void d_unlock_parent_child(int stat, parent, child)
{
	Assert(stat == Error_Unmatch);
	if (stat == Success)
		spin_unlock(&dentry->d_lock);
	spin_unlock(&parent->d_lock);
}


J. R. Okajima
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