Bug about sched_4bsd?

[Kohji Okuno]

Hello,

I have a question.

The sleeping thread (on turnstile or on sleepque) can set sched_lock
to td_lock, kernel are allowed?

Best regards,
 Kohji Okuno.

> Hello,
> 
> Thank you, Attilio.
> I checked your patch. I think that your patch is better.
> I tested the patch quickly, and I think it's OK.
> # This probrem does not occur easily :-<
> 
> 
> What do you think about maybe_resched()?
> I have never experienced about maybe_resched(), but I think that the
> race condition may occur.
> 
> <<Back Trace>>
> sched_4bsd.c:	  maybe_resched()
> sched_4bsd.c:	  resetpriority_thread()
> sched_4bsd.c:	  sched_nice()			get thread_lock(td)
> kern_resource.c:  donice()
> kern_resource.c:  setpriority()			get PROC_LOCK()
> 
> static void
> maybe_resched(struct thread *td)
> {
>         THREAD_LOCK_ASSERT(td, MA_OWNED);
>         if (td->td_priority < curthread->td_priority)
>                 curthread->td_flags |= TDF_NEEDRESCHED;
> }
> 
> I think, when td->td_lock is not &sched_lock, curthread->td_lock is
> not locked in maybe_resched().
> 
> Best regards,
>  Kohji Okuno.
> 
> From: Attilio Rao <attilio at freebsd.org>
> Subject: Re: Bug about sched_4bsd?
> Date: Mon, 18 Jan 2010 03:58:41 +0100
> Message-ID: <3bbf2fe11001171858o4568fe38l9b2db54ec9856b50 at mail.gmail.com>
> 
>> 2010/1/17 Kohji Okuno <okuno.kohji at jp.panasonic.com>:
>>> Hello,
>>>
>>> Could you check sched_4bsd.patch, please?
>> 
>> I think, instead, that what needs to happen is to have sched_switch()
>> to do a lock handover from sleepq/turnstile spinlock to schedlock.
>> That way, if threads are willing to contest on td_lock they will be
>> still inhibited.
>> I'm not sure if this patch breaks any invariant, if you may test I
>> would appreciate:
>> http://www.freebsd.org/~attilio/sched_4bsd_schedlock.diff
>> 
>> Reviews and comments are appreciated.
>> BTW, nice catch.
>> 
>> Attilio
>> 
>> 
>> -- 
>> Peace can only be achieved by understanding - A. Einstein
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