Re: Question: d_revalidate in rcu-walk

[Sheng Yong <shyodx1989@163.com>]

Thanks for your reply. There is something wrong with my mail client. It 
sent an empty email which I don't know why :( I hope it works this time.
在 2016/9/3 21:03, Al Viro 写道:
> On Sat, Sep 03, 2016 at 06:46:05PM +0800, shyodx1989 wrote:
>
>>
>> But most filesystems, which have d_revalidate, return -ECHILD if LOOKUP_RCU is set instead of
>> checking if it is safe for rcu-walk.
>
> For a good and simple reason that the work they would have to do in their
> ->d_revalidate() can't be done without blocking.  Which can't be done under
> rcu_read_lock(), thus the "sorry, you have to leave RCU mode for that", aka
> -ECHILD.

So, basically, ECHILD does not care about wheter d_inode, d_flags or 
other d_xxx members are updated or invalidated. If d_revalidate is not 
blocked, thus the task will not be switched out to break grace period, 
it is safe to call d_revalidate in RCU mode. And during rcu-walk, the 
dentry may be out of date, the seqcount could help to detect if someone 
is updating or using the dentry.

>
>> However commit 5c0f360b083fb33d05d1bff4b138b82d715eb419
>> "jfs_ci_revalidate() is safe from RCU mode" removes the check. So why is jfs_ci_revalidate safe in
>> RCU  mode
>
> Because JFS ->d_revalidate() doesn't need anything blocking.
>
>> and if we only check d_inode (like the following code), what should we care about to
>> tell if d_revalidate is safe for rcu-walk or not and? Ref-walk is much slower than rcu-walk, maybe
>> it's better not to return -ECHILD directly if not necessary.
>>
>>
>> d_revalidate(dentry, flags)
>> {
>>          if (flags & LOOKUP_RCU)
>>                  return -ECHILD
>>          if (!d_inode_rcu(dentry))
>>                  return 0;
>>          return 1;
>> }
>
> Huh?  Which filesystem would that be?  Sure, in such case -ECHILD is pointless;
> who does that?  Some of them might be possible to drop ECHILD, but that needs
> some care.  Note, BTW, that things like dput() are blocking, so the things like
> trying to grab parent, etc. can get tricky.

In fact, there are no filesystems do that. I just wanted to revalidate 
negative dentries during lookup for my own test, and was wondering why 
JFS behaves different with other local filesystems.

thanks,
Sheng

>
> Which ->d_revalidate() instance do you have in mind?
>