From: Jiri Slaby <jslaby@suse.cz>
To: Tejun Heo <tj@kernel.org>, Jiri Kosina <jkosina@suse.cz>
Cc: linux-kernel@vger.kernel.org, jirislaby@gmail.com,
Vojtech Pavlik <vojtech@suse.cz>, Michael Matz <matz@suse.de>,
Steven Rostedt <rostedt@goodmis.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Theodore Ts'o" <tytso@mit.edu>,
Dipankar Sarma <dipankar@in.ibm.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [RFC 09/16] kgr: mark task_safe in some kthreads
Date: Wed, 14 May 2014 16:59:05 +0200 [thread overview]
Message-ID: <537384B9.5090907@suse.cz> (raw)
In-Reply-To: <20140501210943.GB28948@mtj.dyndns.org>
Hi Tejun,
On 05/01/2014 11:09 PM, Tejun Heo wrote:
> On Thu, May 01, 2014 at 05:02:42PM -0400, Tejun Heo wrote:
>> Hello, Jiri.
>>
>> On Thu, May 01, 2014 at 10:17:44PM +0200, Jiri Kosina wrote:
>>> I agree that this expectation might really somewhat implicit and is not
>>> probably properly documented anywhere. The basic observation is "whenever
>>> kthread_should_stop() is being called, all data structures are in a
>>> consistent state and don't need any further updates in order to achieve
>>> consistency, because we can exit the loop immediately here", as
>>> kthread_should_stop() is the very last thing every freezable kernel thread
>>
>> But kthread_should_stop() doesn't necessarily imply that "we can exit
>> the loop *immediately*" at all. It just indicates that it should
>> terminate in finite amount of time. I don't think it'd be too
>
> Just a bit of addition. Please note that kthread_should_stop(), along
> with the freezer test, is actually trickier than it seems. It's very
> easy to write code which works most of the time but misses wake up
> from kill when the timing is just right (or wrong). It should be
> interlocked with set_current_state() and other related queueing data
> structure accesses. This was several years ago but when I audited
> most kthread users in kernel, especially in combination with the
> freezer test which also has similar requirement, surprising percentage
> of users (at least several tens of pct) were getting it slightly
> wrong, so kthread_should_stop() really isn't used as "we can exit
> *immediately*". It just isn't that simple.
I see the worst case scenario. (For curious readers, it is for example
this kthread body:
while (1) {
some_paired_call(); /* invokes pre-patched code */
if (kthread_should_stop()) { /* kgraft switches to the new code */
its_paired_function(); /* invokes patched code (wrong) */
break;
}
its_paired_function(); /* the same (wrong) */
})
What to do with that now? We have come up with a couple possibilities.
Would you consider try_to_freeze() a good state-defining function? As it
is called when a kthread expects weird things can happen, it should be
safe to switch to the patched version in our opinion.
The other possibility is to patch every kthread loop (~300) and insert
kgr_task_safe() semi-manually at some proper place.
Or if you have any other suggestions we would appreciate that?
thanks,
--
js
suse labs
next prev parent reply other threads:[~2014-05-14 14:59 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-30 14:30 [RFC 00/16] kGraft Jiri Slaby
2014-04-30 14:30 ` [RFC 01/16] ftrace: Add function to find fentry of function Jiri Slaby
2014-04-30 14:48 ` Steven Rostedt
2014-04-30 14:58 ` Jiri Slaby
2014-04-30 14:30 ` [RFC 02/16] ftrace: Make ftrace_is_dead available globally Jiri Slaby
2014-04-30 14:30 ` [RFC 03/16] kgr: initial code Jiri Slaby
2014-04-30 14:56 ` Steven Rostedt
2014-04-30 14:57 ` Jiri Slaby
2014-05-01 20:20 ` Andi Kleen
2014-05-01 20:37 ` Jiri Kosina
2014-05-14 9:28 ` Aravinda Prasad
2014-05-14 10:12 ` Jiri Slaby
2014-05-14 10:41 ` Aravinda Prasad
2014-05-14 10:44 ` Jiri Slaby
2014-05-14 11:19 ` Aravinda Prasad
2014-05-20 11:36 ` Jiri Slaby
2014-05-21 18:28 ` Aravinda Prasad
2014-05-26 8:50 ` Jiri Kosina
2014-04-30 14:30 ` [RFC 04/16] kgr: add testing kgraft patch Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-05-12 12:50 ` Jiri Slaby
2014-04-30 14:30 ` [RFC 05/16] kgr: update Kconfig documentation Jiri Slaby
2014-05-03 14:32 ` Randy Dunlap
2014-04-30 14:30 ` [RFC 06/16] kgr: add Documentation Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-05-09 9:31 ` kgr: dealing with optimalizations? (was Re: [RFC 06/16] kgr: add Documentat)ion Pavel Machek
2014-05-09 12:22 ` Michael Matz
2014-04-30 14:30 ` [RFC 07/16] kgr: trigger the first check earlier Jiri Slaby
2014-04-30 14:30 ` [RFC 08/16] kgr: sched.h, introduce kgr_task_safe helper Jiri Slaby
2014-04-30 14:30 ` [RFC 09/16] kgr: mark task_safe in some kthreads Jiri Slaby
2014-04-30 15:49 ` Greg Kroah-Hartman
2014-04-30 16:55 ` Paul E. McKenney
2014-04-30 18:33 ` Vojtech Pavlik
2014-04-30 19:07 ` Paul E. McKenney
2014-05-01 14:24 ` Tejun Heo
2014-05-01 20:17 ` Jiri Kosina
2014-05-01 21:02 ` Tejun Heo
2014-05-01 21:09 ` Tejun Heo
2014-05-14 14:59 ` Jiri Slaby [this message]
2014-05-14 15:15 ` Vojtech Pavlik
2014-05-14 15:30 ` Paul E. McKenney
2014-05-14 16:32 ` Tejun Heo
2014-05-15 3:53 ` Mike Galbraith
2014-05-15 4:06 ` Tejun Heo
2014-05-15 4:46 ` Mike Galbraith
2014-05-15 4:50 ` Tejun Heo
2014-05-15 5:04 ` Mike Galbraith
2014-05-15 5:09 ` Tejun Heo
2014-05-15 5:32 ` Mike Galbraith
2014-05-15 6:05 ` Tejun Heo
2014-05-15 6:32 ` Mike Galbraith
2014-04-30 14:30 ` [RFC 10/16] kgr: kthreads support Jiri Slaby
2014-04-30 14:30 ` [RFC 11/16] kgr: handle irqs Jiri Slaby
2014-04-30 14:30 ` [RFC 12/16] kgr: add tools Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-04-30 14:30 ` [RFC 13/16] kgr: add MAINTAINERS entry Jiri Slaby
2014-04-30 14:30 ` [RFC 14/16] kgr: x86: refuse to build without fentry support Jiri Slaby
2014-04-30 14:30 ` [RFC 15/16] kgr: add procfs interface for per-process 'kgr_in_progress' Jiri Slaby
2014-04-30 14:30 ` [RFC 16/16] kgr: make a per-process 'in progress' flag a single bit Jiri Slaby
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