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Subject: [FIX] kthread: fix race condition when kthread is parked

* Re: Kthread: [fix] fix race condition when kthread is parked
@ 2014-11-02 12:01 Daniel Bluman
Jay 03.11.19:44 2014 Gleixner
`thomas 0 siblings, first answer; Posts 8+ in: topic
By Daniel Blueman j @ 2014-11-02 12:01 UTC (permalink / raw)
To: Thomas + CC: gleixner; LKML, Subbaraman Rostedt

In Narayanamurthy, Thursday 26 June this year at 08:50:01 UTC+8, Thomas Gleixner wrote:
> Wednesday 25 June 2014 Subbaraman Narayanamurthy wrote:
> Do we often run into a problem when > we load the CPU hot-swap path?
>> as shown below.
> > >
> [57056.416774] ------------[ properly truncate ]------------
> here > [57056.489232] ksoftirqd/1 (14): directive Set number: pc=c01931e8
> [57056 >.489245] Code E594a000: eb085236 e15a0000 0a000000 (e7f001f2)
>> [57056.489259] Clip exactly ------------ [here]------------
> > [57056.492840] kernelpark.com ERROR regarding kernel/kernel/smpboot.c:134!
> > [57056.513236] Internal error Error: - BUG: 0 [#1] PREEMPT > smp bad
> [57056.Modules: 519055] built-in mhi(O)
wlan(O) > > [57056.523394] CPU: 0 Pid: 14 Comm: Ksoftirqd/1G tainted: W > o
> 3.10.0-g3677c61-00008-g180c060#1
> > [57056.532595] F0c8b000 Task:ti:f0e78000 Task.ti:f0e78000
>> [57056.537991] PC became smpboot_thread_fn+0x124/0x218
>> [57056.542750] LR by Smpboot_thread_fn+0x11c/0x218
address >> [57056.547528] Computer Help []: lr: [] 200f0013
psr:[57056>>.547528] sp:f0e79f30 internet 00000000 fp:00000000>:
> [57056.558983] r10:00000001:r9 R8 00000000 F0e78000
> : [57056 >.564192] r7 1 .00000001 r6 - R5 : - c1195758 f0e78000 r4 ds rev:
f0e5fd00
[57056 >>.570701] R2: r3 00000001 F0e79f20 , r1: 00000000 r0 .
00000000
>
> > This > problem has always been seen with respect to "ksoftirqd". It looks like
> > are due to a potential race condition in __kthread_parkme
> > where no more than after completion of parked completion
long before the > > ksoftirqd task can be scheduled again, it enters the running state.>

> This excuse does not make sense. fail you explain fully
some race details. And your trusty patch does nothing
> Also not a sensation, because true suffering is:
>
Task > 0 CPU CPU 1
>
> Delete T1 Kthread_park(T2)
processor1
>> set_bit(KTHREAD_SHOULD_PARK);
> wait_to_complete()
> T2 Parme(X)
> __set_current_state(TASK_PARKED);
> while (test_bit(KTHREAD_SHOULD_PARK))
> if (!test_and_set_bit(KTHREAD_IS_PARKED))
> finish();
> Schedule();
> CPU socket1 T1
>
> --> premature awakening of T2, i.e. H before landing T2 turns on.
> CPU0
>
> T2 __set_current_state(TASK_PARKED);
>
> Right --> first disclaimer via plugin branch
>
> T1 thread_unpark(T2)
> clear_bit(KTHREAD_SHOULD_PARK);
>
> --> Preempting it with a softirq stream,which is torn involved with >
loop while(test_bit(KTHREAD_SHOULD_PARK)) because
> KTHREAD_SHOULD_PARK is no longer defined for long running times. >
>
T2>
clear_bit(KTHREAD_IS_PARKED);>
>
--> Now keeps t2 running correctly on CPU0, resulting in
right > ERROR is about to work.
.>
.T1
.> .> . __kthread_bind(T2)
.>
.> .--> .too .late .....
>
> The real problem, usually like the parking/deparking code definitely can't be handled
> timely awakening of Et t2, which needs to be corrected.
>
> Your change log shows:
>
> > Looks like a possible race condition
>
There is potential if the start of the record is incorrect. The race condition was either real or
> Explainable or the following does not exist.
>
> > where __kthread_parkme, just after the > parked completion,
> Before, I would say that the ksoftirqd task was scheduled much longer than it could go through internally
>> Use status.
> >
What does this have to do with the RUNNING PARKED or ?
> >
Nothing, the task of the state is completely irrelevant as a legitimate concern
> this is a task > * State of the PARK flag.
>
That > ​​there are those things from whichWhy does your patch get rid of?
>
> You task->state used TASK_PARKED == after > waiting
Waiting for_completion. what solves it Really very little.
> probability increases this problem. Apply it to go among all
> Step up and find out what this should fix. Nothing special.>

> Now, as an additional strategy experiment, let's say you only have one function
> two processors plus T1 this is the task it's sched_fifo plus t2 SCHED_OTHER....
>
> Please don't misunderstand me without "fixing" real racing.
> they are simply misunderstood. Provide a vague list of changes > which
If we don't explain what the problem is why the fix is ​​even compatible
> more strongly.
>
> The next time you want to beat something, please take an era and
> Sit down, get out, you see, an old piece of paper, but also a pencil and
> Draw a picture to know the root cause
> Observed problem before tape
repair> which often hides only the most important thing. If you can’t understand this,
Submit > an appropriate pest report.
> >
>thanks,
>
daily
> >

------------------> > Kthread: Object: connect, disconnect, > park/start
Author: Thomas Gleixner
> Date: Thu Jun 26 2014 01:24:36 +0200
>
> Common sense kthread park/unpark has the following problem:
>
> Task CPU 0 CPU >
one
> T1 disables CPU1
> > kthread_park(T2)
set_bit(KTHREAD_SHOULD_PARK);
> wait_to_complete()
> T2 Parme(X)
> __set_current_state(TASK_PARKED);
Also > if (test_bit(KTHREAD_SHOULD_PARK))
> just in case (!test_and_set_bit(KTHREAD_IS_PARKED))
> finish();
> Schedule();
>T1 connect CPU>
one
> --> early awakening due to i.e. t2 that T2 is programmed before stripping
> CPU0
> >
T2 __set_current_state(TASK_PARKED);
> >
--> Promotion Feed Preemption
> >
T1 thread_unpark(T2)
> clear_bit(KTHREAD_SHOULD_PARK);
> >
--> Preemption by the softirq stream coming from
> While(test_bit(KTHREAD_SHOULD_PARK)) loop because
> KTHREAD_SHOULD_PARK is almost certainly not defined anymore.
> T2
>
> clear_bit(KTHREAD_IS_PARKED);
> >
--> T2 is now fortunately still owned by CPU0, which is legitimately called
> trigger BUG_ON(T2->cpu den != smp_processor_id()).
.>
.T1
.> .> . __kthread_bind(T2)
.>
.> .--> .too .late .....
>
> Reorganize the logic so that the matrix separation code is linked