On Mon, Sep 14, 2015 at 08:51:08PM +0000, Serge Hallyn wrote:
> Quoting Christian Brauner (christianvanbrau...@gmail.com):
> > On Mon, Sep 14, 2015 at 08:31:59PM +0000, Serge Hallyn wrote:
> > > Quoting Christian Brauner (christianvanbrau...@gmail.com):
> > > > On Mon, Sep 14, 2015 at 07:27:06PM +0000, Serge Hallyn wrote:
> > > > > Quoting Christian Brauner (christianvanbrau...@gmail.com):
> > > > > > On Mon, Sep 14, 2015 at 04:33:05PM +0000, Serge Hallyn wrote:
> > > > > > > Quoting Serge Hallyn (serge.hal...@ubuntu.com):
> > > > > > > > Quoting Christian Brauner (christianvanbrau...@gmail.com):
> > > > > > > > > On Mon, Sep 14, 2015 at 02:50:39PM +0000, Serge Hallyn wrote:
> > > > > > > > > > Quoting Christian Brauner (christianvanbrau...@gmail.com):
> > > > > > > > > > > When creating ephemeral containers that have the option
> > > > > > > > > > > lxc.ephemeral = 1 set
> > > > > > > > > > > in their config, they will be destroyed on shutdown. As
> > > > > > > > > > > they are simple overlay
> > > > > > > > > > > clones of an existing container they should be registered
> > > > > > > > > > > in the lxc_snapshots
> > > > > > > > > > > file of the original container to stay consistent and
> > > > > > > > > > > adhere to the
> > > > > > > > > > > expectancies of the users. Most of all, it ensure that we
> > > > > > > > > > > cannot remove a
> > > > > > > > > > > container that has clones, even if they are just
> > > > > > > > > > > ephemeral snapshot-clones. The
> > > > > > > > > > > function adds further consistency because
> > > > > > > > > > > remove_snapshots_entry() ensures that
> > > > > > > > > > > ephemeral clone-snapshots deregister themselves from the
> > > > > > > > > > > lxc_snapshots file
> > > > > > > > > > > when they are destroyed.
> > > > > > > > > > >
> > > > > > > > > > > POSSIBLE GLITCH:
> > > > > > > > > > > I was thinking hard about racing conditions and
> > > > > > > > > > > concurrent acces on the
> > > > > > > > > > > lxc_snapshots file when lxc-destroy is called on the
> > > > > > > > > > > container while we
> > > > > > > > > > > shutdown then container from inside. Here is what my
> > > > > > > > > > > thoughts are so far:
> > > > > > > > > > >
> > > > > > > > > > > There should be no racing condition when lxc-destroy
> > > > > > > > > > > including all snapshots is
> > > > > > > > > >
> > > > > > > > > > Note that lxcapi_destroy_with_snapshots() deletes the
> > > > > > > > > > *snapshots*, not the
> > > > > > > > > > snapshot clones. This is an unfortunate naming clash
> > > > > > > > > > (which we could try
> > > > > > > > > > to correct henceforth but we need good names :), but they
> > > > > > > > > > are different.
> > > > > > > > > > So anything under /var/lib/lxc/$container/snaps will be
> > > > > > > > > > deleted. But if
> > > > > > > > > > you've created an overlayfs clone, then containers listed in
> > > > > > > > > > /var/lib/lxc/$container/lxc_snapshots will not be deleted.
> > > > > > > > > > There is no
> > > > > > > > > > API call or program to automatically deleted those right
> > > > > > > > > > now.
> > > > > > > > >
> > > > > > > > > I think you are partially wrong here. I was not thinking
> > > > > > > > > about problems created
> > > > > > > > > by an API-call but by the lxc-destroy exectuable. A quick
> > > > > > > > > walkthrough: With the
> > > > > > > >
> > > > > > > > D'oh. Yeah, you'll need to mutex that somehow.
> > > > > > >
> > > > > > > If you want help up-front with the design, please let me know.
> > > > > > > If you
> > > > > > > aren't sure what the current container_disk_lock() and
> > > > > > > container_mem_lock()
> > > > > > > do, please shout. (they are explained in a LOCKING comment above
> > > > > > > lxc_container_free() in src/lxc/lxccontainer.c)
> > > > > > >
> > > > > > > The easiest thing to do mght be to disk_lock the container in
> > > > > > > lxc_destroy.c,
> > > > > > > then make the mod_rdep() helper which you use in lxc_destroy.c be
> > > > > > > a _locked
> > > > > > > variant (to avoid deadlock). So mod_rdep() would turn into
> > > > > > > something like:
> > > > > > >
> > > > > > > static bool mod_rdep(struct lxc_container *c0, struct
> > > > > > > lxc_container *c, bool inc)
> > > > > > > {
> > > > > > > bool ret;
> > > > > > > if (container_disk_lock(c0))
> > > > > > > return false;
> > > > > > > ret = mod_rdep_locked(c0->name, c0->lxcpath, c->name,
> > > > > > > c->lxcpath);
> > > > > > > container_disk_unlock(c0);
> > > > > > > return ret;
> > > > > > > }
> > > > > > >
> > > > > > > -serge
> > > > > >
> > > > > > Thanks, this is really nice! One question:
> > > > > > - I'll take it that we want to make mod_rdep() public. mod_rdep()
> > > > > > will be used
> > > > > > in lxc_destroy.c, lxccontainer.c and start.c. Problem is that in
> > > > > > start.c we do
> > > > > > not have a container to pass into mod_rdep(). Do you want me to
> > > > > > rewrite
> > > > > > mod_rdep() to take in lxcpath and lxcname? If so, could we still
> > > > > > use
> > > > >
> > > > > Ok, I'm not running on all cylinders, sorry.
> > > > >
> > > > > You don't want mod_rdep, you want mod_all_rdeps. So yes export that,
> > > > > make a struct lxc_container, lock it, and pass that to mod_all_rdeps
> > > > > which will dothe mod_rdeps for you.
> > > >
> > > > Ok, so in start.c we can actually use lxc_container new. I could modify
> > > > my:
> > >
> > > Yeah I think that looks good,
> >
> > Then one final question:
> > Should we make mod_all_rdeps() public with lxccontainer.h or should we move
> > it
> > to a different header?
>
> Not lxccontainer.h, because that would mean we want programs using liblxc
> to call it directly, which we don't. We want it to be done automatically when
> needed. Something like utils.h would be good.
The easiest way to achieve this would be to #include lxccontainer.h in utils.h
because we need mod_all_rdep() multiple times in lxccontainer.c and start.c and
mod_all_rdep() needs struct lxc_container declared. Unless I'm missing something
terribly obvious. Fine with that? Or rather some extern trickery? Or other
ideas?
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