Date: Mon, 14 Jan 2013 14:03:45 -0500 From: John Baldwin <jhb@freebsd.org> To: attilio@freebsd.org Cc: FreeBSD Current <freebsd-current@freebsd.org>, Ryan Stone <rysto32@gmail.com> Subject: Re: Spurious witness warning when destroying spin mtx Message-ID: <201301141403.45905.jhb@freebsd.org> In-Reply-To: <CAJ-FndDL18oQdFZQh4AKr9NbOc2WxWJoDVjOtkjt%2Bb7w36E_kA@mail.gmail.com> References: <CAFMmRNyYccyXFh0r2jC2Q5ynYQH09SiZNguLp8X4JWSX4Lua5w@mail.gmail.com> <CAJ-FndDL18oQdFZQh4AKr9NbOc2WxWJoDVjOtkjt%2Bb7w36E_kA@mail.gmail.com>
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On Saturday, November 24, 2012 10:01:39 am Attilio Rao wrote: > On Sat, Nov 24, 2012 at 3:08 AM, Ryan Stone <rysto32@gmail.com> wrote: > > Today I saw a spurious witness warning for "acquiring duplicate lock of > > same type". The root cause is that when running mtx_destroy on a spinlock > > that is held by the current thread, mtx_destroy calls spinlock_exit() > > before calling WITNESS_UNLOCK, which opens up a window in which the CPU can > > be interrupted and attempt to acquire another spinlock of the same type as > > the one being destroyed. This patch should fix it: > > I seriously wonder why right now we don't assume the lock is unheld. > There are likely historically reasons for that, but I would like to > know which one are those and eventually fix them out. > FWIK, all the other locking primitives assume the lock is already > unheld when destroying and I think it would be good to have that for > mutexes as well. That is simply behavior we inherited from BSD/OS. I didn't find it all that useful so all of the other locking primitives I've added since then have not had this behavior. -- John Baldwin
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