Re: [PATCH 08/22] fsnotify: Attach marks to object via dedicated head structure

[Jan Kara <jack@suse.cz>]

On Fri 23-12-16 14:34:07, Jan Kara wrote:
> On Fri 23-12-16 07:48:43, Amir Goldstein wrote:
> > On Thu, Dec 22, 2016 at 11:15 AM, Jan Kara <jack@suse.cz> wrote:
> > > Currently notification marks are attached to object (inode or vfsmnt) by
> > > a hlist_head in the object. The list is also protected by a spinlock in
> > > the object. So while there is any mark attached to the list of marks,
> > > the object must be pinned in memory (and thus e.g. last iput() deleting
> > > inode cannot happen). Also for list iteration in fsnotify() to work, we
> > > must hold fsnotify_mark_srcu lock so that mark itself and
> > > mark->obj_list.next cannot get freed. Thus we are required to wait for
> > > response to fanotify events from userspace process with
> > > fsnotify_mark_srcu lock held. That causes issues when userspace process
> > > is buggy and does not reply to some event - basically the whole
> > > notification subsystem gets eventually stuck.
> > >
> > > So to be able to drop fsnotify_mark_srcu lock while waiting for
> > > response, we have to pin the mark in memory and make sure it stays in
> > > the object list (as removing the mark waiting for response could lead to
> > > lost notification events for groups later in the list). However we don't
> > > want inode reclaim to block on such mark as that would lead to system
> > > just locking up elsewhere.
> > >
> > > This commit tries to pave a way towards solving these conflicting
> > > lifetime needs. Instead of anchoring the list of marks directly in the
> > > object, we anchor it in a dedicated structure (fsnotify_mark_list) and
> > > just point to that structure from the object. Also the list is protected
> > > by a spinlock contained in that structure. With this, we can detach
> > > notification marks from object without having to modify the list itself.
> > >
> > 
> > The structural change looks very good to.
> > It makes the code much easier to manage IMO.
> > 
> > I am only half way though this big change, but I wanted to make one meta
> > comment.
> > 
> > I have a problem with the choice of naming for the new struct.
> > 'list' is really an overloaded term and the use of 'list' as a name of
> > a class that
> > contains a list head makes for some really confusing constructs like
> > list->list and mark->obj_list_head which is not a list_head struct.
> 
> OK, I'll think about better naming. I agree it may be slightly confusing.

So how about naming the type fsnotify_mark_connector? We can use 'conn' as
a name for local variables. I think that is not as overloaded as 'list'
and it describes that it is a structure used for connecting marks with
inode / vfsmount. Would that make things more comprehensive for you?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR