Re: [PATCH] fs-pin: allow pin_remove() to be called other than from ->kill()

[Kinglong Mee <kinglongmee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 8/18/2015 14:21, NeilBrown wrote:
> On Tue, 18 Aug 2015 14:07:58 +0800 Kinglong Mee <kinglongmee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> wrote:
> 
>> Sorry for my so late reply.
>>
>> On 7/29/2015 11:59, NeilBrown wrote:
>>> fs-pin currently assumes when either the vfsmount or the fs_pin wants
>>> to unpin, pin_kill() will be called.
>>> This requires that the ->kill() function can wait for any transient
>>> references to the fs_pin to be released.  If the structure containing
>>> the fs_pin doesn't already have the ability to wait for references,
>>> this can be a burden.
>>>
>>> As the fs_pin already has infrastructure for waiting, that can be
>>> leveraged to remove the burden.
>>>
>>> In this alternate scenario, only the vfsmount calls pin_kill() when it
>>> wants to unpin.  The owner of the fs_pin() instead calls pin_remove().
>>>
>>> The ->kill() function removes any long-term references, and then calls
>>> pin_kill() (recursively).
>>> When the last reference on (the structure containing) the fs_pin is
>>> dropped, pin_remove() will be called and the (recursive) pin_kill()
>>> call will complete.
>>>
>>> For this to be safe, the final "put" must *not* free the structure if
>>> pin_kill() has already been called, as that could leave ->kill()
>>> accessing freed data.
>>>
>>> So we provide a return value for pin_remove() which reports the old
>>> ->done value.
>>>
>>> When final put calls pin_remove() it checks that value.
>>> If it was 0, then pin_kill() has not called ->kill and will not,
>>> so final put can free the data structure.
>>> If it was -1, then pin_kill() has called ->kill, and ->kill will
>>> free the data structure - final put must not touch it.
>>
>> I find another problem, 
>> how can xxx_pin_kill known the last reference of the data have be put?
>>
>> eg,
>> static void expkey_pin_kill(struct fs_pin *pin)
>> {
>>         struct svc_expkey *key = container_of(pin, struct svc_expkey, ek_pin);
>>         cache_delete_entry(key->cd, &key->h);
>>         expkey_destroy(key);
>> }
>>
>> expkey_pin_kill has call cache_delete_entry() but doesn't know whether
>> the last reference has be put (here expkey_put is called)? 
>>
>> Before the cache_list is deleted from the cache, a third user gets
>> the reference, so that, the third user will be the last put of the cache
>> by calling expkey_put, xxx_pin_kill only decrease the reference.
> 
> expkey_pin_kill() should call:
>   cache_delete_entry()
>   pin_kill()
>   expkey_destroy()
> 
> The "cache_delete_entry()" call removes the only long-term reference.
> Any other reference will be transient so it is safe to wait for those.
> 
> The 'pin_kill()' call will wait of pin_remove() to be called (it
> already does that).

Sorry for my missing of calling pin_kill() here.

> pin_remove() will be called when the last reference is dropped.  As
> described above, that pin_remove call will return -1 and so the 'put'
> function will not have called expkey_destroy.
> 
> Finally the expkey_destroy() function actually frees the data
> structure.  No other code can be touching at this point.

With calling pin_kill() again in expkey_pin_kill makes every clear now.
Thanks again.

The only thing is waiting Al's opinion. 

thanks,
Kinglong Mee

> 
> Thanks,
> NeilBrown
> 
> 
>>
>> thanks,
>> Kinglong Mee
>>
>>>
>>> This makes the 'wait' infrastructure of fs_pin available to any
>>> pinning client which wants to use it.
>>>
>>> Signed-Off-By: NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>
>>>
>>> ---
>>> Hi Al,
>>>  do you see this as a workable solution?  I think it will improve the nfsd pining patch
>>> a lot.
>>>
>>> Thanks,
>>> NeilBrown
>>>
>>>
>>> diff --git a/fs/fs_pin.c b/fs/fs_pin.c
>>> index 611b5408f6ec..b7954a9d17da 100644
>>> --- a/fs/fs_pin.c
>>> +++ b/fs/fs_pin.c
>>> @@ -6,16 +6,32 @@
>>>  
>>>  static DEFINE_SPINLOCK(pin_lock);
>>>  
>>> -void pin_remove(struct fs_pin *pin)
>>> +/**
>>> + * pin_remove - disconnect an fs_pin from the pinned structure.
>>> + * @pin:	The struct fs_pin which is pinning something.
>>> + *
>>> + * Detach a 'pin' which was added by pin_insert().  A return value
>>> + * of -1 implies that pin_kill() has already been called and that the
>>> + * ->kill() function now owns the data structure containing @pin.
>>> + * The function which called pin_remove() must not touch the data structure
>>> + * again (unless it is the ->kill() function itself).
>>> + * A return value of 0 implies an uneventful disconnect: pin_kill() has not called,
>>> + * and will not call, the ->kill() function on this @pin.
>>> + * Any other return value is a usage error - e.g. repeated call to pin_remove().
>>> + */
>>> +int pin_remove(struct fs_pin *pin)
>>>  {
>>> +	int ret;
>>>  	spin_lock(&pin_lock);
>>>  	hlist_del_init(&pin->m_list);
>>>  	hlist_del_init(&pin->s_list);
>>>  	spin_unlock(&pin_lock);
>>>  	spin_lock_irq(&pin->wait.lock);
>>> +	ret = pin->done;
>>>  	pin->done = 1;
>>>  	wake_up_locked(&pin->wait);
>>>  	spin_unlock_irq(&pin->wait.lock);
>>> +	return ret;
>>>  }
>>>  
>>>  void pin_insert_group(struct fs_pin *pin, struct vfsmount *m, struct hlist_head *p)
>>> diff --git a/include/linux/fs_pin.h b/include/linux/fs_pin.h
>>> index 3886b3bffd7f..2fe9d3ba09e8 100644
>>> --- a/include/linux/fs_pin.h
>>> +++ b/include/linux/fs_pin.h
>>> @@ -18,7 +18,7 @@ static inline void init_fs_pin(struct fs_pin *p, void (*kill)(struct fs_pin *))
>>>  	p->kill = kill;
>>>  }
>>>  
>>> -void pin_remove(struct fs_pin *);
>>> +int pin_remove(struct fs_pin *);
>>>  void pin_insert_group(struct fs_pin *, struct vfsmount *, struct hlist_head *);
>>>  void pin_insert(struct fs_pin *, struct vfsmount *);
>>>  void pin_kill(struct fs_pin *);
>>>
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