From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
Andrew Cooper <andrew.cooper3@citrix.com>,
George Dunlap <george.dunlap@citrix.com>,
Julien Grall <julien@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [PATCH v3 1/6] xen: add reference counter support
Date: Thu, 16 Mar 2023 17:48:46 +0100 [thread overview]
Message-ID: <ZBNIbhW3Eu07aj8I@Air-de-Roger> (raw)
In-Reply-To: <8331a0c4-07fa-0340-20bf-77a5c8661aa1@suse.com>
On Thu, Mar 16, 2023 at 05:43:18PM +0100, Jan Beulich wrote:
> On 16.03.2023 17:39, Roger Pau Monné wrote:
> > On Thu, Mar 16, 2023 at 05:32:38PM +0100, Jan Beulich wrote:
> >> On 16.03.2023 17:19, Roger Pau Monné wrote:
> >>> On Tue, Mar 14, 2023 at 08:56:29PM +0000, Volodymyr Babchuk wrote:
> >>>> +static inline void refcnt_get(refcnt_t *refcnt)
> >>>> +{
> >>>> + int old = atomic_add_unless(&refcnt->refcnt, 1, 0);
> >>>
> >>> Occurred to me while looking at the next patch:
> >>>
> >>> Don't you also need to print a warning (and saturate the counter
> >>> maybe?) if old == 0, as that would imply the caller is attempting
> >>> to take a reference of an object that should be destroyed? IOW: it
> >>> would point to some kind of memory leak.
> >>
> >> Hmm, I notice the function presently returns void. I think what to do
> >> when the counter is zero needs leaving to the caller. See e.g.
> >> get_page() which will simply indicate failure to the caller in case
> >> the refcnt is zero. (There overflow handling also is left to the
> >> caller ... All that matters is whether a ref can be acquired.)
> >
> > Hm, likely. I guess pages never go away even when it's refcount
> > reaches 0.
> >
> > For the pdev case attempting to take a refcount on an object that has
> > 0 refcounts implies that the caller is using leaked memory, as the
> > point an object reaches 0 it supposed to be destroyed.
>
> Hmm, my thinking was that a device would remain at refcnt 0 until it is
> actually removed, i.e. refcnt == 0 being a prereq for pci_remove_device()
> to be willing to do anything at all. But maybe that's not a viable model.
Right, I think the intention was for pci_remove_device() to drop the
refcount to 0 and do the removal, so the refcount should be 1 when
calling pci_remove_device(). But none of this is written down, so
it's mostly my assumptions from looking at the code.
I have some comments about the model in patch 2, I think we need to
clarify the intended usage on the commit log about pdev and refcounts.
Thanks, Roger.
next prev parent reply other threads:[~2023-03-16 16:49 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-14 20:56 [PATCH v3 0/6] vpci: first series in preparation for vpci on ARM Volodymyr Babchuk
2023-03-14 20:56 ` [PATCH v3 1/6] xen: add reference counter support Volodymyr Babchuk
2023-03-16 13:54 ` Roger Pau Monné
2023-03-16 14:03 ` Jan Beulich
2023-03-16 16:21 ` Roger Pau Monné
2023-04-11 22:27 ` Volodymyr Babchuk
2023-04-12 10:12 ` Roger Pau Monné
2023-03-16 16:19 ` Roger Pau Monné
2023-03-16 16:32 ` Jan Beulich
2023-03-16 16:39 ` Roger Pau Monné
2023-03-16 16:43 ` Jan Beulich
2023-03-16 16:48 ` Roger Pau Monné [this message]
2023-03-16 16:56 ` Jan Beulich
2023-03-17 10:05 ` Roger Pau Monné
2023-03-17 14:46 ` Jan Beulich
2023-03-16 17:01 ` Jan Beulich
2023-04-11 22:38 ` Volodymyr Babchuk
2023-04-17 6:47 ` Jan Beulich
2023-03-14 20:56 ` [PATCH v3 2/6] xen: pci: introduce reference counting for pdev Volodymyr Babchuk
2023-03-16 16:16 ` Roger Pau Monné
2023-03-29 9:55 ` Jan Beulich
2023-03-29 10:48 ` Roger Pau Monné
2023-03-29 11:58 ` Jan Beulich
2023-04-11 23:41 ` Volodymyr Babchuk
2023-04-12 9:13 ` Roger Pau Monné
2023-04-12 21:54 ` Volodymyr Babchuk
2023-04-13 15:00 ` Roger Pau Monné
2023-04-14 1:30 ` Volodymyr Babchuk
2023-04-17 10:17 ` Roger Pau Monné
2023-04-17 10:34 ` Jan Beulich
2023-04-17 10:51 ` Roger Pau Monné
2023-04-17 11:02 ` Jan Beulich
2023-04-21 11:00 ` Volodymyr Babchuk
2023-04-21 12:24 ` Jan Beulich
2023-04-21 13:02 ` Volodymyr Babchuk
2023-04-21 13:10 ` Roger Pau Monné
2023-04-21 14:13 ` Volodymyr Babchuk
2023-04-24 7:46 ` Jan Beulich
2023-04-24 14:15 ` Volodymyr Babchuk
2023-04-24 14:27 ` Jan Beulich
2023-03-29 10:04 ` Jan Beulich
2023-03-14 20:56 ` [PATCH v3 3/6] vpci: crash domain if we wasn't able to (un) map vPCI regions Volodymyr Babchuk
2023-03-16 16:32 ` Roger Pau Monné
2023-03-14 20:56 ` [PATCH v3 6/6] xen: pci: print reference counter when dumping pci_devs Volodymyr Babchuk
2023-03-17 8:46 ` Roger Pau Monné
2023-03-14 20:56 ` [PATCH v3 5/6] vpci: use reference counter to protect vpci state Volodymyr Babchuk
2023-03-17 8:43 ` Roger Pau Monné
2023-03-29 9:31 ` Jan Beulich
2023-03-14 20:56 ` [PATCH v3 4/6] vpci: restrict unhandled read/write operations for guests Volodymyr Babchuk
2023-03-17 8:37 ` Roger Pau Monné
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZBNIbhW3Eu07aj8I@Air-de-Roger \
--to=roger.pau@citrix.com \
--cc=Volodymyr_Babchuk@epam.com \
--cc=andrew.cooper3@citrix.com \
--cc=george.dunlap@citrix.com \
--cc=jbeulich@suse.com \
--cc=julien@xen.org \
--cc=sstabellini@kernel.org \
--cc=wl@xen.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).