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Date: Mon, 23 Jul 2007 12:20:27 -0700
From: Nate Lawson <nate@root.org>
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To: John Baldwin <jhb@freebsd.org>
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Cc: cvs-src@freebsd.org, src-committers@freebsd.org, cvs-all@freebsd.org,
	Andrew Thompson <thompsa@freebsd.org>
Subject: Re: cvs commit: src/sys/compat/ndis subr_ntoskrnl.c
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John Baldwin wrote:
> On Sunday 22 July 2007 04:53:28 pm Andrew Thompson wrote:
>> thompsa     2007-07-22 20:53:28 UTC
>>
>>   FreeBSD src repository
>>
>>   Modified files:
>>     sys/compat/ndis      subr_ntoskrnl.c 
>>   Log:
>>   ndis will signal the kthread to exit and then sleep on the proc pointer to
>>   be woken up by kthread_exit. This is racey and in some cases the kthread 
> will
>>   exit before ndis gets around to sleep so it will be stuck indefinitely. 
> This
>>   change reuses the kq_exit variable to indicate that the thread has gone 
> and
>>   will loop on tsleep with a timeout waiting for it. If the kthread has 
> already
>>   exited then it will not sleep at all.
> 
> As long as you use a lock you are ok.  That is:
> 
> foo_detach()
> {
> 
> 	mtx_lock(&lock);
> 	please_die = 1;
> 	msleep(&proc, &lock, ..., 0);
> 	mtx_unlock(&lock);
> }
> 
> foo_main()
> {
> 
> 	mtx_lock(&lock);
> 	while (!please_die) {
> 		do_stuff();
> 	}
> 	mtx_unlock(&lock);
> 	kthread_exit(0);
> }
> 
> works fine.

How can foo_detach() acquire the lock to set "please_die" if foo_main()
always holds it?  Are you assuming that "do_stuff()" drops the lock
somewhere internally?

-Nate