From: Michal Hocko <mhocko@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-acpi@vger.kernel.org,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Andrew Morton <akpm@linux-foundation.org>,
Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init()
Date: Thu, 25 Jul 2019 21:19:43 +0200 [thread overview]
Message-ID: <20190725191943.GA6142@dhcp22.suse.cz> (raw)
In-Reply-To: <447b74ca-f7c7-0835-fd50-a9f7191fe47c@redhat.com>
On Thu 25-07-19 16:35:07, David Hildenbrand wrote:
> On 25.07.19 15:57, Michal Hocko wrote:
> > On Thu 25-07-19 15:05:02, David Hildenbrand wrote:
> >> On 25.07.19 14:56, Michal Hocko wrote:
> >>> On Wed 24-07-19 16:30:17, David Hildenbrand wrote:
> >>>> We end up calling __add_memory() without the device hotplug lock held.
> >>>> (I used a local patch to assert in __add_memory() that the
> >>>> device_hotplug_lock is held - I might upstream that as well soon)
> >>>>
> >>>> [ 26.771684] create_memory_block_devices+0xa4/0x140
> >>>> [ 26.772952] add_memory_resource+0xde/0x200
> >>>> [ 26.773987] __add_memory+0x6e/0xa0
> >>>> [ 26.775161] acpi_memory_device_add+0x149/0x2b0
> >>>> [ 26.776263] acpi_bus_attach+0xf1/0x1f0
> >>>> [ 26.777247] acpi_bus_attach+0x66/0x1f0
> >>>> [ 26.778268] acpi_bus_attach+0x66/0x1f0
> >>>> [ 26.779073] acpi_bus_attach+0x66/0x1f0
> >>>> [ 26.780143] acpi_bus_scan+0x3e/0x90
> >>>> [ 26.780844] acpi_scan_init+0x109/0x257
> >>>> [ 26.781638] acpi_init+0x2ab/0x30d
> >>>> [ 26.782248] do_one_initcall+0x58/0x2cf
> >>>> [ 26.783181] kernel_init_freeable+0x1bd/0x247
> >>>> [ 26.784345] kernel_init+0x5/0xf1
> >>>> [ 26.785314] ret_from_fork+0x3a/0x50
> >>>>
> >>>> So perform the locking just like in acpi_device_hotplug().
> >>>
> >>> While playing with the device_hotplug_lock, can we actually document
> >>> what it is protecting please? I have a bad feeling that we are adding
> >>> this lock just because some other code path does rather than with a good
> >>> idea why it is needed. This patch just confirms that. What exactly does
> >>> the lock protect from here in an early boot stage.
> >>
> >> We have plenty of documentation already
> >>
> >> mm/memory_hotplug.c
> >>
> >> git grep -C5 device_hotplug mm/memory_hotplug.c
> >>
> >> Also see
> >>
> >> Documentation/core-api/memory-hotplug.rst
> >
> > OK, fair enough. I was more pointing to a documentation right there
> > where the lock is declared because that is the place where people
> > usually check for documentation. The core-api documentation looks quite
> > nice. And based on that doc it seems that this patch is actually not
> > needed because neither the online/offline or cpu hotplug should be
> > possible that early unless I am missing something.
>
> I really prefer to stick to locking rules as outlined on the
> interfaces if it doesn't hurt. Why it is not needed is not clear.
>
> >
> >> Regarding the early stage: primarily lockdep as I mentioned.
> >
> > Could you add a lockdep splat that would be fixed by this patch to the
> > changelog for reference?
> >
>
> I have one where I enforce what's documented (but that's of course not
> upstream and therefore not "real" yet)
Then I suppose to not add locking for something that is not a problem.
Really, think about it. People will look at this code and follow the
lead without really knowing why the locking is needed.
device_hotplug_lock has its purpose and if the code in question doesn't
need synchronization for the documented scenarios then the locking
simply shouldn't be there. Adding the lock just because of a
non-existing, and IMHO dubious, lockdep splats is just wrong.
We need to rationalize the locking here, not to add more hacks.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2019-07-25 19:19 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-24 14:30 [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init() David Hildenbrand
2019-07-25 9:11 ` Rafael J. Wysocki
2019-07-25 9:18 ` Oscar Salvador
2019-07-25 9:22 ` Rafael J. Wysocki
2019-07-25 9:23 ` David Hildenbrand
2019-07-25 12:56 ` Michal Hocko
2019-07-25 13:05 ` David Hildenbrand
2019-07-25 13:57 ` Michal Hocko
2019-07-25 14:35 ` David Hildenbrand
2019-07-25 19:19 ` Michal Hocko [this message]
2019-07-25 20:49 ` David Hildenbrand
2019-07-25 21:23 ` Rafael J. Wysocki
2019-07-26 7:20 ` David Hildenbrand
2019-07-26 7:57 ` Michal Hocko
2019-07-26 8:05 ` David Hildenbrand
2019-07-26 8:31 ` Michal Hocko
2019-07-26 8:36 ` David Hildenbrand
2019-07-26 8:44 ` Michal Hocko
2019-07-26 8:57 ` David Hildenbrand
2019-07-26 10:31 ` Michal Hocko
2019-07-26 10:37 ` David Hildenbrand
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=20190725191943.GA6142@dhcp22.suse.cz \
--to=mhocko@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=david@redhat.com \
--cc=linux-acpi@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=osalvador@suse.de \
--cc=rjw@rjwysocki.net \
/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).