Return-Path: Received: from mx2.suse.de ([195.135.220.15]:36739 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751202AbbHRGVv (ORCPT ); Tue, 18 Aug 2015 02:21:51 -0400 Date: Tue, 18 Aug 2015 16:21:39 +1000 From: NeilBrown To: Kinglong Mee Cc: Al Viro , "J. Bruce Fields" , "linux-nfs@vger.kernel.org" , linux-fsdevel@vger.kernel.org, Trond Myklebust Subject: Re: [PATCH] fs-pin: allow pin_remove() to be called other than from ->kill() Message-ID: <20150818162139.3c214136@noble> In-Reply-To: <55D2CBBE.9080807@gmail.com> References: <55B5A012.1030006@gmail.com> <55B5A186.7040004@gmail.com> <20150729135914.13cb0f86@noble> <55D2CBBE.9080807@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-nfs-owner@vger.kernel.org List-ID: On Tue, 18 Aug 2015 14:07:58 +0800 Kinglong Mee 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 > > > > --- > > 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 *); > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html