From: Petr Mladek <pmladek@suse.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: Jiri Kosina <jikos@kernel.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Jason Baron <jbaron@akamai.com>,
Joe Lawrence <joe.lawrence@redhat.com>,
Jessica Yu <jeyu@kernel.org>,
Evgenii Shatokhin <eshatokhin@virtuozzo.com>,
live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v12 00/12]
Date: Thu, 11 Oct 2018 14:48:28 +0200 [thread overview]
Message-ID: <20181011124828.bc2reaplfisvergs@pathway.suse.cz> (raw)
In-Reply-To: <alpine.LSU.2.21.1808301353170.18557@pobox.suse.cz>
On Thu 2018-08-30 13:58:15, Miroslav Benes wrote:
> On Tue, 28 Aug 2018, Petr Mladek wrote:
>
> > livepatch: Atomic replace feature
> >
> > The atomic replace allows to create cumulative patches. They
> > are useful when you maintain many livepatches and want to remove
> > one that is lower on the stack. In addition it is very useful when
> > more patches touch the same function and there are dependencies
> > between them.
> >
> > This version does another big refactoring based on feedback against
> > v11[*]. In particular, it removes the registration step, changes
> > the API and handling of livepatch dependencies. The aim is
> > to keep the number of possible variants on a sane level.
> > It helps the keep the feature "easy" to use and maintain.
> >
> > [*] https://lkml.kernel.org/r/20180323120028.31451-1-pmladek@suse.com
>
> Hi,
>
> I've started to review the patch set. Running selftests with lockdep
> enabled gives me...
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 4.17.0-rc1-klp_replace_v12-117114-gfedb3eba611d #218 Tainted: G
> K
> ------------------------------------------------------
> kworker/1:1/49 is trying to acquire lock:
> 00000000bb88dc17 (kn->count#186){++++}, at: kernfs_remove+0x23/0x40
>
> but task is already holding lock:
> 0000000073632424 (klp_mutex){+.+.}, at: klp_transition_work_fn+0x17/0x40
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (klp_mutex){+.+.}:
> lock_acquire+0xd4/0x220
> __mutex_lock+0x75/0x920
> mutex_lock_nested+0x1b/0x20
> enabled_store+0x47/0x150
> kobj_attr_store+0x12/0x20
> sysfs_kf_write+0x4a/0x60
> kernfs_fop_write+0x123/0x1b0
> __vfs_write+0x2b/0x150
> vfs_write+0xc7/0x1c0
> ksys_write+0x49/0xa0
> __x64_sys_write+0x1a/0x20
> do_syscall_64+0x62/0x1b0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #0 (kn->count#186){++++}:
> __lock_acquire+0xe9d/0x1240
> lock_acquire+0xd4/0x220
> __kernfs_remove+0x23c/0x2c0
> kernfs_remove+0x23/0x40
> sysfs_remove_dir+0x51/0x60
> kobject_del+0x18/0x50
> kobject_cleanup+0x4b/0x180
> kobject_put+0x2a/0x50
> __klp_free_patch+0x5b/0x60
> klp_free_patch_nowait+0x12/0x30
> klp_try_complete_transition+0x13e/0x180
> klp_transition_work_fn+0x26/0x40
> process_one_work+0x1d8/0x5d0
> worker_thread+0x4d/0x3d0
> kthread+0x113/0x150
> ret_from_fork+0x3a/0x50
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(klp_mutex);
> lock(kn->count#186);
> lock(klp_mutex);
> lock(kn->count#186);
Sigh, I overestimated the power of kobjects. I thought that this
must have been a false positive but it was not.
1. kernfs_fop_write() ignores kobj->kref. It takes care only
of its own reference count, see kernfs_get_active().
2. kobj_put() takes care only of kobj->kref. The following
code is called when the reference count reaches zero:
+ kobj_put()
+ kref_put()
+ kobject_release()
+ kobject_cleanup()
+ kobject_del()
+ sysfs_remove_dir()
+ kernfs_remove()
+ __kernfs_remove().
+ kernfs_drain()
, where kernfs_drain() waits until all opened files
are closed.
Now, we call kobj_put() under klp_mutex() when the sysfs
interface still exists. Files can be opened for
writing. As a result:
+ enabled_store() might wait for klp_mutex
+ kernfs_drain() would wait for enabled_store()
with klp_mutex() taken.
I have reproduced this with some extra sleeps.
I am going to work on another solution.
Best Regards,
Petr
prev parent reply other threads:[~2018-10-11 12:48 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-28 14:35 [PATCH v12 00/12] Petr Mladek
2018-08-28 14:35 ` [PATCH v12 01/12] livepatch: Change void *new_func -> unsigned long new_addr in struct klp_func Petr Mladek
2018-08-31 8:37 ` Miroslav Benes
2018-08-28 14:35 ` [PATCH v12 02/12] livepatch: Helper macros to define livepatch structures Petr Mladek
2018-08-28 14:35 ` [PATCH v12 03/12] livepatch: Shuffle klp_enable_patch()/klp_disable_patch() code Petr Mladek
2018-08-31 8:38 ` Miroslav Benes
2018-08-28 14:35 ` [PATCH v12 04/12] livepatch: Consolidate klp_free functions Petr Mladek
2018-08-31 10:39 ` Miroslav Benes
2018-10-12 11:43 ` Petr Mladek
2018-08-28 14:35 ` [PATCH v12 05/12] livepatch: Refuse to unload only livepatches available during a forced transition Petr Mladek
2018-08-28 14:35 ` [PATCH v12 06/12] livepatch: Simplify API by removing registration step Petr Mladek
2018-09-05 9:34 ` Miroslav Benes
2018-10-12 13:01 ` Petr Mladek
2018-10-15 16:01 ` Miroslav Benes
2018-10-18 14:54 ` Petr Mladek
2018-10-18 15:30 ` Josh Poimboeuf
2018-10-19 12:16 ` Miroslav Benes
2018-10-19 14:36 ` Josh Poimboeuf
2018-10-22 13:25 ` Petr Mladek
2018-10-23 16:39 ` Josh Poimboeuf
2018-10-24 2:55 ` Josh Poimboeuf
2018-10-24 11:14 ` Petr Mladek
2018-08-28 14:35 ` [PATCH v12 07/12] livepatch: Use lists to manage patches, objects and functions Petr Mladek
2018-09-03 16:00 ` Miroslav Benes
2018-10-12 12:12 ` Petr Mladek
2018-08-28 14:35 ` [PATCH v12 08/12] livepatch: Add atomic replace Petr Mladek
2018-08-28 14:36 ` [PATCH v12 09/12] livepatch: Remove Nop structures when unused Petr Mladek
2018-09-04 14:50 ` Miroslav Benes
2018-08-28 14:36 ` [PATCH v12 10/12] livepatch: Atomic replace and cumulative patches documentation Petr Mladek
2018-09-04 15:15 ` Miroslav Benes
2018-08-28 14:36 ` [PATCH v12 11/12] livepatch: Remove ordering and refuse loading conflicting patches Petr Mladek
2018-08-28 14:36 ` [PATCH v12 12/12] selftests/livepatch: introduce tests Petr Mladek
2018-08-30 11:58 ` [PATCH v12 00/12] Miroslav Benes
2018-10-11 12:48 ` Petr Mladek [this message]
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=20181011124828.bc2reaplfisvergs@pathway.suse.cz \
--to=pmladek@suse.com \
--cc=eshatokhin@virtuozzo.com \
--cc=jbaron@akamai.com \
--cc=jeyu@kernel.org \
--cc=jikos@kernel.org \
--cc=joe.lawrence@redhat.com \
--cc=jpoimboe@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=live-patching@vger.kernel.org \
--cc=mbenes@suse.cz \
/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).