Date: Fri, 23 Nov 2012 22:08:28 -0500 From: Ryan Stone <rysto32@gmail.com> To: FreeBSD Current <freebsd-current@freebsd.org> Subject: Spurious witness warning when destroying spin mtx Message-ID: <CAFMmRNyYccyXFh0r2jC2Q5ynYQH09SiZNguLp8X4JWSX4Lua5w@mail.gmail.com>
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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: diff --git a/sys/kern/kern_mutex.c b/sys/kern/kern_mutex.c index 2f13863..96f43f8 100644 --- a/sys/kern/kern_mutex.c +++ b/sys/kern/kern_mutex.c @@ -918,16 +918,16 @@ _mtx_destroy(volatile uintptr_t *c) else { MPASS((m->mtx_lock & (MTX_RECURSED|MTX_CONTESTED)) == 0); + lock_profile_release_lock(&m->lock_object); + /* Tell witness this isn't locked to make it happy. */ + WITNESS_UNLOCK(&m->lock_object, LOP_EXCLUSIVE, __FILE__, + __LINE__); + /* Perform the non-mtx related part of mtx_unlock_spin(). */ if (LOCK_CLASS(&m->lock_object) == &lock_class_mtx_spin) spinlock_exit(); else curthread->td_locks--; - - lock_profile_release_lock(&m->lock_object); - /* Tell witness this isn't locked to make it happy. */ - WITNESS_UNLOCK(&m->lock_object, LOP_EXCLUSIVE, __FILE__, - __LINE__); } m->mtx_lock = MTX_DESTROYED
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