Re: overlayfs patches for ovl_copy_up & ovl_rename

[Jordi Pujol <jordipujolp@gmail.com>]

A Dimarts 17 Maig 2011 20:07:13, vàreu escriure:
> Jordi, what about the original bug you reported:
> >>>- a few days ago we talked about an error when repetitive sed
> >>>instructions were executed, also it happens in other processes, like
> >>>mantaining list of installed files for actual packages,
> >>>that is solved in ovl_rename issuing a dget for the old-dentry and
> >>>parent dentries. (This code is inspired on rename.c from unionfs)
> 
> Is this still happening?  Does the debug patch make any difference?

my patch adds some dget/dput operations that minimize the problem, it occurs 
less times, but it happens.
I suppose that those extra operations add an execution delay before processing 
the main routine;
a deduction is that locking dentries is required to synchronize a rename to 
not be executed until another rename operation has finished.

As an example of code that looks like that of unionfs, (this is not tested) 

struct ovl_entry {
	struct dentry *__upperdentry;
	struct dentry *lowerdentry;
+	struct mutex d_mutex;
	union {
		struct {
			u64 version;
			bool opaque;
		};
		struct rcu_head rcu;
	};
};

static struct ovl_entry *ovl_alloc_entry(void)
{
	struct ovl_entry *oe;

	oe = kzalloc(sizeof(struct ovl_entry), GFP_KERNEL);
	mutex_init(&oe->d_mutex);

	return oe;
}

void ovl_lock_dentry(struct dentry *dentry)
{
	struct ovl_entry *oe = dentry->d_fsdata;

	mutex_lock(&oe->d_mutex);
}

void ovl_unlock_dentry(struct dentry *dentry)
{
	struct ovl_entry *oe = dentry->d_fsdata;

	mutex_unlock(&oe->d_mutex);
}

static int ovl_rename(struct inode *olddir, struct dentry *old,
			struct inode *newdir, struct dentry *new)
{
	int err;
...........
+	ovl_lock_dentry(old);

	err = ovl_copy_up(old);
...........
+	ovl_unlock_dentry(old);
	return err;
}

consider this idea,

Thanks,

Jordi Pujol

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