Return-Path: Received: from mail-pa0-f49.google.com ([209.85.220.49]:33753 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750791AbbHRGhb (ORCPT ); Tue, 18 Aug 2015 02:37:31 -0400 Subject: Re: [PATCH] fs-pin: allow pin_remove() to be called other than from ->kill() To: NeilBrown References: <55B5A012.1030006@gmail.com> <55B5A186.7040004@gmail.com> <20150729135914.13cb0f86@noble> <55D2CBBE.9080807@gmail.com> <20150818162139.3c214136@noble> Cc: Al Viro , "J. Bruce Fields" , "linux-nfs@vger.kernel.org" , linux-fsdevel@vger.kernel.org, Trond Myklebust , kinglongmee@gmail.com From: Kinglong Mee Message-ID: <55D2D29E.30706@gmail.com> Date: Tue, 18 Aug 2015 14:37:18 +0800 MIME-Version: 1.0 In-Reply-To: <20150818162139.3c214136@noble> Content-Type: text/plain; charset=windows-1252 Sender: linux-nfs-owner@vger.kernel.org List-ID: On 8/18/2015 14:21, NeilBrown wrote: > 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). 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 >>> >>> --- >>> 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 > >