Date: Sat, 24 Nov 2012 15:01:39 +0000 From: Attilio Rao <attilio@freebsd.org> To: Ryan Stone <rysto32@gmail.com>, John Baldwin <jhb@freebsd.org> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: Spurious witness warning when destroying spin mtx Message-ID: <CAJ-FndDL18oQdFZQh4AKr9NbOc2WxWJoDVjOtkjt%2Bb7w36E_kA@mail.gmail.com> In-Reply-To: <CAFMmRNyYccyXFh0r2jC2Q5ynYQH09SiZNguLp8X4JWSX4Lua5w@mail.gmail.com> References: <CAFMmRNyYccyXFh0r2jC2Q5ynYQH09SiZNguLp8X4JWSX4Lua5w@mail.gmail.com>
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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. Can you please show which lock triggers the panic you saw? Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein
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