schedlock 4/4

Don't hold the scheduler lock while doing context switches.  Instead we
unlock after selecting the new thread and switch within a spinlock
section leaving interrupts and preemption disabled to prevent local
concurrency.  This means that mi_switch() is entered with the thread
locked but returns without.  This dramatically simplifies scheduler
locking because we will not hold the schedlock while spinning on
blocked lock in switch.

This change has not been made to 4BSD but in principle it would be
more straightforward.

Discussed with:	markj
Reviewed by:	kib
Tested by:	pho
Differential Revision: https://reviews.freebsd.org/D22778

1 files changed, 7 insertions, 7 deletions

diff --git a/sys/kern/subr_sleepqueue.c b/sys/kern/subr_sleepqueue.c
index 6e7410c4f46f..bd5150c1134b 100644
--- a/sys/kern/subr_sleepqueue.c
+++ b/sys/kern/subr_sleepqueue.c
@@ -546,8 +546,10 @@ out:
 		sq = sleepq_lookup(wchan);
 		sleepq_remove_thread(sq, td);
 	}
-	mtx_unlock_spin(&sc->sc_lock);
 	MPASS(td->td_lock != &sc->sc_lock);
+	mtx_unlock_spin(&sc->sc_lock);
+	thread_unlock(td);
+
 	return (ret);
 }
 
@@ -574,6 +576,7 @@ sleepq_switch(void *wchan, int pri)
 	 */
 	if (td->td_sleepqueue != NULL) {
 		mtx_unlock_spin(&sc->sc_lock);
+		thread_unlock(td);
 		return;
 	}
 
@@ -605,6 +608,7 @@ sleepq_switch(void *wchan, int pri)
 		sq = sleepq_lookup(wchan);
 		sleepq_remove_thread(sq, td);
 		mtx_unlock_spin(&sc->sc_lock);
+		thread_unlock(td);
 		return;
 	}
 #ifdef SLEEPQUEUE_PROFILING
@@ -616,7 +620,7 @@ sleepq_switch(void *wchan, int pri)
 	thread_lock_set(td, &sc->sc_lock);
 	SDT_PROBE0(sched, , , sleep);
 	TD_SET_SLEEPING(td);
-	mi_switch(SW_VOL | SWT_SLEEPQ, NULL);
+	mi_switch(SW_VOL | SWT_SLEEPQ);
 	KASSERT(TD_IS_RUNNING(td), ("running but not TDS_RUNNING"));
 	CTR3(KTR_PROC, "sleepq resume: thread %p (pid %ld, %s)",
 	    (void *)td, (long)td->td_proc->p_pid, (void *)td->td_name);
@@ -668,7 +672,6 @@ sleepq_wait(void *wchan, int pri)
 	MPASS(!(td->td_flags & TDF_SINTR));
 	thread_lock(td);
 	sleepq_switch(wchan, pri);
-	thread_unlock(td);
 }
 
 /*
@@ -681,7 +684,6 @@ sleepq_wait_sig(void *wchan, int pri)
 	int rcatch;
 
 	rcatch = sleepq_catch_signals(wchan, pri);
-	thread_unlock(curthread);
 	if (rcatch)
 		return (rcatch);
 	return (sleepq_check_signals());
@@ -698,9 +700,9 @@ sleepq_timedwait(void *wchan, int pri)
 
 	td = curthread;
 	MPASS(!(td->td_flags & TDF_SINTR));
+
 	thread_lock(td);
 	sleepq_switch(wchan, pri);
-	thread_unlock(td);
 
 	return (sleepq_check_timeout());
 }
@@ -715,8 +717,6 @@ sleepq_timedwait_sig(void *wchan, int pri)
 	int rcatch, rvalt, rvals;
 
 	rcatch = sleepq_catch_signals(wchan, pri);
-	thread_unlock(curthread);
-
 	/* We must always call check_timeout() to clear sleeptimo. */
 	rvalt = sleepq_check_timeout();
 	rvals = sleepq_check_signals();