From: "Roger Pau Monné" <roger.pau@citrix.com>
To: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>,
Wei Liu <wei.liu2@citrix.com>,
xen-devel@lists.xen.org
Subject: Re: [PATCH] libxl: trigger attach events for devices attached before xl devd startup
Date: Mon, 11 Jul 2016 12:00:02 +0200 [thread overview]
Message-ID: <20160711100002.uiguv74u3iqlzmsg@mac> (raw)
In-Reply-To: <20160711094919.GQ19103@mail-itl>
On Mon, Jul 11, 2016 at 11:49:19AM +0200, Marek Marczykowski-Górecki wrote:
> On Mon, Jul 11, 2016 at 11:43:18AM +0200, Roger Pau Monné wrote:
> > On Mon, Jul 11, 2016 at 10:56:04AM +0200, Marek Marczykowski-Górecki wrote:
> > > On Mon, Jul 11, 2016 at 10:31:17AM +0200, Roger Pau Monné wrote:
> > > > On Sun, Jul 10, 2016 at 07:35:47PM +0200, Marek Marczykowski-Górecki wrote:
> > > > > When this daemon is started after creating backend device, that device
> > > > > will not be configured.
> > > > >
> > > > > Racy situation:
> > > > > 1. driver domain is started
> > > > > 2. frontend domain is started (just after kicking driver domain off)
> > > > > 3. device in frontend domain is connected to the backend (as specified
> > > > > in frontend domain configuration)
> > > > > 4. xl devd is started in driver domain
> > > > >
> > > > > End result is that backend device in driver domain is not configured
> > > > > (like network interface is not enabled), so the device doesn't work.
> > > > >
> > > > > Fix this by artifically triggering events for devices already present in
> > > > > xenstore before xl devd is started. Do this only after xenstore watch is
> > > > > already registered, and only for devices not already initialized (in
> > > > > XenbusStateInitWait state).
> > > >
> > > > Thanks!
> > > >
> > > > > Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> > > > > Cc: Wei Liu <wei.liu2@citrix.com>
> > > > > Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> > > > > ---
> > > > > tools/libxl/libxl.c | 40 ++++++++++++++++++++++++++++++++++++++++
> > > > > 1 file changed, 40 insertions(+)
> > > > >
> > > > > diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> > > > > index 1c81239..99815a7 100644
> > > > > --- a/tools/libxl/libxl.c
> > > > > +++ b/tools/libxl/libxl.c
> > > > > @@ -4743,8 +4743,16 @@ int libxl_device_events_handler(libxl_ctx *ctx,
> > > > > uint32_t domid;
> > > > > libxl__ddomain ddomain;
> > > > > char *be_path;
> > > > > + char **kinds = NULL, **domains = NULL, **devs = NULL;
> > > > > + const char *sstate;
> > > > > + char *state_path;
> > > > > + int state;
> > > > > + unsigned int nkinds, ndomains, ndevs;
> > > > > + int i, j, k;
> > > > > + xs_transaction_t t;
> > > > >
> > > > > ddomain.ao = ao;
> > > > > + FILLZERO(ddomain.watch);
> > > >
> > > > Is this a different bugfix or stray change?
> > >
> > > To cleanly unregister watch and not do nothing if wasn't registered at
> > > all. If it isn't initialized, libxl__ev_xswatch_deregister call on
> > > not registered watch isn't harmless.
> >
> > Right, I've realized that before your changes the function only registered
> > the watch and exited, this is needed now.
> >
> > > > > LIBXL_SLIST_INIT(&ddomain.guests);
> > > > >
> > > > > rc = libxl__get_domid(gc, &domid);
> > > > > @@ -4762,9 +4770,41 @@ int libxl_device_events_handler(libxl_ctx *ctx,
> > > > > be_path);
> > > > > if (rc) goto out;
> > > > >
> > > > > + rc = libxl__xs_transaction_start(gc, &t);
> > > > > + if (rc) goto out;
> > > >
> > > > Why do you need to start a transaction here if you end up aborting it when
> > > > finished?
> > >
> > > Mostly to ease error checking. Because below code does three level
> > > listing, I don't want to deal with races where some entry was removed
> > > between those calls, at least not here. Like this:
> > >
> > > xs_directory('backend/vif') -> 3, 4, 5
> > > xs_directory('backend/vif/3') -> 0, 1
> > > xs_read('backend/vif/3/0/state') -> ...
> > > xs_read('backend/vif/3/1/state') -> ...
> > > toolstack removes backend/vif/4 here
> > > xs_directory('backend/vif/4') -> fail
> > >
> > > Of course backend_watch_callback would fail anyway in such a case, which
> > > is ok. But having snapshot of xenstore during this multi-level listing
> > > looks like avoiding some corner cases during listing itself.
> >
> > AFAICT your code seems to be prepared to deal with entries disappearing in
> > the middle of the tree walk, so I would just remove the transaction.
>
> Actually I'm considering changing error handling below to "goto out"
> instead of "continue", as race condition should be eliminated by the
> transaction, other errors (permission denied for example) maybe should
> be considered fatal. So, IMO there two options:
> - ignore failed reads and remove transaction
> - error on failed reads and keep transaction
>
> Which one would be better?
IMHO, I think the first option is better, and you already have it coded.
Roger.
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next prev parent reply other threads:[~2016-07-11 10:00 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-10 17:35 [PATCH] libxl: trigger attach events for devices attached before xl devd startup Marek Marczykowski-Górecki
2016-07-11 8:31 ` Roger Pau Monné
2016-07-11 8:56 ` Marek Marczykowski-Górecki
2016-07-11 9:43 ` Roger Pau Monné
2016-07-11 9:49 ` Marek Marczykowski-Górecki
2016-07-11 10:00 ` Roger Pau Monné [this message]
2016-07-11 10:44 ` [PATCH v2] " Marek Marczykowski-Górecki
2016-07-11 10:53 ` Roger Pau Monné
2016-07-14 9:36 ` Wei Liu
2016-07-15 23:47 ` [PATCH v3] " Marek Marczykowski-Górecki
2016-07-18 15:31 ` Wei Liu
2016-07-19 13:20 ` Wei Liu
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