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SubjectRe: [PATCH v2 2/2] cgroup: fix cgroup_path() vs rename() race, take 2
On 2013/2/19 1:30, Tejun Heo wrote:
> Hello, Li.
>
> On Mon, Feb 18, 2013 at 09:16:48AM +0800, Li Zefan wrote:
>> @@ -171,6 +171,7 @@ struct cgroup {
>>
>> struct cgroup *parent; /* my parent */
>> struct dentry *dentry; /* cgroup fs entry, RCU protected */
>> + char __rcu *name; /* a copy of dentry->d_name */
>
> A brief explanation of why this is necessary and how rcu is used would
> be nice.
>

The comments in cgroup_path() explains why we can't use dentry->d_name,
which suggests why cgrp->name is needed. I'll revise the comments there,
and add comments on the rcu thing.

>> +static char *cgroup_alloc_name(struct dentry *dentry)
>> +{
>> + char *name;
>> +
>> + name = kmalloc(dentry->d_name.len + 1, GFP_KERNEL);
>> + if (!name)
>> + return NULL;
>> + memcpy(name, dentry->d_name.name, dentry->d_name.len);
>> + name[dentry->d_name.len] = '\0';
>> + return name;
>> +}
>
> While d_name has length field, it's always properly NULL terminated,
> so kstrdup() should suffice here. Right, Al?
>

Oh you're right. We pass dentry->d_name.name to printk %s in other places.

>> @@ -1613,13 +1626,19 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
> ...
>> - inode = sb->s_root->d_inode;
>> + dentry = sb->s_root;
>> + inode = dentry->d_inode;
>> +
>> + root_cgrp->name = cgroup_alloc_name(dentry);
>> + if (!root_cgrp->name)
>> + goto drop_new_super;
>
> Don't we need an RCU assignment? Is it safe because it isn't online
> yet? But wouldn't this still trigger sparse warning?
>

Yeah, it's safe.

To be frank, I haven't used sparse for years. Will check.

>> @@ -1751,6 +1770,8 @@ static void cgroup_kill_sb(struct super_block *sb) {
>> mutex_unlock(&cgroup_root_mutex);
>> mutex_unlock(&cgroup_mutex);
>>
>> + synchronize_rcu();
>
> An explanation on what we're synchronizing would be nice. Barriers
> without explanation sucks because there's nothing directly linking the
> barriers to the things which are being protected.
>
>> @@ -2539,13 +2558,41 @@ static int cgroup_file_release(struct inode *inode, struct file *file)
>> static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
>> struct inode *new_dir, struct dentry *new_dentry)
>> {
> ...
>> + old_name = cgrp->name;
>> + rcu_assign_pointer(cgrp->name, name);
>> +
>> + synchronize_rcu();
>
> Please don't call synchronize_rcu() from interface which is directly
> visible to userland. It leads to sporadic difficult-to-reproduce
> latencies which hurt enough in corner cases and this is kmalloc
> memory. It's not like kfree_rcu() is difficult to use or anything.
>

ok

>> + kfree(old_name);
>> + return 0;
>> }
>>
>> static struct simple_xattrs *__d_xattrs(struct dentry *dentry)
>> @@ -4144,9 +4191,13 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>> if (!cgrp)
>> return -ENOMEM;
>>
>> + cgrp->name = cgroup_alloc_name(dentry);
>> + if (!cgrp->name)
>> + goto err_free_cgrp;
>
> Ditto with assignment.
>
> Thanks.
>



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