Re: Bug about sched_4bsd?

Hello,

Could you check sched_4bsd.patch, please?

--
Best regards,
 Kohji Okuno.

> Hello, 
> 
> I think that sched_4bsd.c:sched_switch() has a problem.
> 
> 
> I encounterd the problem that a process could not exit. 
> That process has multi threads and p_flag was as below.
> 
>  p_flag : (P_INMEM|P_STOPPED_SINGLE|P_EXEC|P_SINGLE_EXIT|P_PPWAIT|P_CONTROLT)
> 
> 
> One thread (THREAD A) was suspended as below.
>  sched_switch()
>  mi_switch()
>  thread_suspend_switch()
>  thread_single()
>  exit1()
>  sys_exit()
> 
>  td_flags : TDF_NEEDSIGCHK|TDF_ASTPENDING|TDF_CANSWAP|TDF_INMEM
> 
> 
> Another thread (THREAD B) was sleeping as below.
>  sched_switch()
>  mi_switch()
>  sleepq_switch()
>  sleepq_catch_signals()
>  sleepq_wait_sig()
>  _cv_wait_sig()
>  seltdwait()
>  kern_select()
>  select()
> 
>  td_flags : TDF_CANSWAP|TDF_SINTR|TDF_INMEM
> 
> 
> That process could not exit, because THREAD B did not execute
> "thread_suspend_check()" and THREAD A had been suspended.
> 
> 
> I think that the race condition had occurred about td_flags as below.
> 
> In sched_switch(), when td->td_lock is not &sched_lock, td->td_lock
> will be unlocked as shown below. And then, td->td_flags will change
> without td->td_lock.
> 
> 
> <<THREAD A>>
>   kern_thread.c:
>   int
>   thread_single(int mode)
>   {
>           ...
> 
>           thread_lock(td2);
>           td2->td_flags |= TDF_ASTPENDING | TDF_NEEDSUSPCHK;
>           
>    *** I think that td2 specified THREAD B in this time.
>       
> <<THREAD B>>
>   sched_4bsd.c:
>   void
>   sched_switch(struct thread *td, struct thread *newtd, int flags)
>   {
>           ...
> 
>           /*
>            * Switch to the sched lock to fix things up and pick
>            * a new thread.
>            */
>           if (td->td_lock != &sched_lock) {
>                   mtx_lock_spin(&sched_lock);
>                   thread_unlock(td);
>           }
> 
>    *** I think that td_lock was sleepqueue_chagin->sc_lock.
>           ...
> 
>           td->td_lastcpu = td->td_oncpu;
>           td->td_flags &= ~TDF_NEEDRESCHED;
>           td->td_owepreempt = 0;
>           td->td_oncpu = NOCPU;
> 
> --
> Thanks,
>  Kohji Okuno.
> 
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