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

From: NeilBrown <neilb@suse.com>
To: Kinglong Mee <kinglongmee@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org,
	Trond Myklebust <trond.myklebust@primarydata.com>
Subject: Re: [PATCH] fs-pin: allow pin_remove() to be called other than from ->kill()
Date: Tue, 18 Aug 2015 16:21:39 +1000	[thread overview]
Message-ID: <20150818162139.3c214136@noble> (raw)
In-Reply-To: <55D2CBBE.9080807@gmail.com>

On Tue, 18 Aug 2015 14:07:58 +0800 Kinglong Mee <kinglongmee@gmail.com>
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).
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.

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@suse.com>
> > 
> > ---
> > 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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