mtx: add mtx_spin_wait_unlocked

The primitive can be used to wait for the lock to be released. Intended
usage is for locks in structures which are about to be freed.

The benefit is the avoided interrupt enable/disable trip + atomic op to
grab the lock and shorter wait if the lock is held (since there is no
worry someone will contend on the lock, re-reads can be more aggressive).

Briefly discussed with:	 kib

1 files changed, 2 insertions, 0 deletions

diff --git a/sys/sys/mutex.h b/sys/sys/mutex.h
index 805df8adc40a..f9ce4e3bdca7 100644
--- a/sys/sys/mutex.h
+++ b/sys/sys/mutex.h
@@ -125,6 +125,8 @@ int	__mtx_trylock_spin_flags(volatile uintptr_t *c, int opts,
 	     const char *file, int line);
 void	__mtx_unlock_spin_flags(volatile uintptr_t *c, int opts,
 	    const char *file, int line);
+void	mtx_spin_wait_unlocked(struct mtx *m);
+
 #if defined(INVARIANTS) || defined(INVARIANT_SUPPORT)
 void	__mtx_assert(const volatile uintptr_t *c, int what, const char *file,
 	    int line);