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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