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

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

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.

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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