Date: Tue, 26 May 2009 14:37:48 -0400 From: John Baldwin <jhb@freebsd.org> To: Kip Macy <kmacy@freebsd.org> Cc: src-committers@freebsd.org, svn-src-user@freebsd.org Subject: Re: svn commit: r192604 - in user/kmacy/releng_7_2_fcs/sys: dev/hwpmc sys Message-ID: <200905261437.49148.jhb@freebsd.org> In-Reply-To: <3c1674c90905261118y282d9b0fn7d54630f507d8363@mail.gmail.com> References: <200905222145.n4MLjhm3019802@svn.freebsd.org> <200905260830.36831.jhb@freebsd.org> <3c1674c90905261118y282d9b0fn7d54630f507d8363@mail.gmail.com>
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On Tuesday 26 May 2009 2:18:00 pm Kip Macy wrote: > On Tue, May 26, 2009 at 5:30 AM, John Baldwin <jhb@freebsd.org> wrote: > > On Friday 22 May 2009 5:45:43 pm Kip Macy wrote: > >> Author: kmacy > >> Date: Fri May 22 21:45:43 2009 > >> New Revision: 192604 > >> URL: http://svn.freebsd.org/changeset/base/192604 > >> > >> Log: > >> - remove pmc_kthread_mtx > >> - replace sleep/wakeup on kthread with a condvar > > > > If you do not sleep on the kthread/kproc when waiting for a kthread to > > exit, then you open up a race window. Usually this matters more for > > any kthreads that live in a module since the race allows the module to > > be unmapped before the thread is finished executing code from the module > > resulting in a fatal page fault in the kernel. > > > Yes, that is why the kthread explicitly calls cv_signal after clearing > the kthread pointer. Umm, that doesn't help. Your kthread's main routine is "physically" located in your kld. It needs to get into the text of kthread_exit() before it is safe for your module to unload, and so you have to sleep on the kthread/kproc pointer to wait for the wakeup in kthread_exit() to fire. You could have easily used msleep_spin() for this w/o hacking up condvar's to support spin locks too, FWIW. -- John Baldwinhelp
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