From a09e3660685d853a4df332437f7aee9222b1ab7f Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Wed, 29 Jan 2020 16:46:10 +0000
Subject: [PATCH] Fix lockstat setup and teardown synchronization.

Use xc_barrier() to enforce memory ordering on all CPUs so there's no
need for membar_producer or atomic_load_acquire.  Avoid bodgy tsleep.
Omit needless membars.
---
 external/cddl/osnet/dev/lockstat/lockstat.c | 15 ++++------
 sys/dev/lockstat.c                          | 33 ++++++++++++++-------
 sys/dev/lockstat.h                          | 11 +++----
 3 files changed, 33 insertions(+), 26 deletions(-)

diff --git a/external/cddl/osnet/dev/lockstat/lockstat.c b/external/cddl/osnet/dev/lockstat/lockstat.c
index 53e6997e0236..21ce216ec2f8 100644
--- a/external/cddl/osnet/dev/lockstat/lockstat.c
+++ b/external/cddl/osnet/dev/lockstat/lockstat.c
@@ -149,13 +149,6 @@ static dtrace_pops_t lockstat_pops = {
 	lockstat_destroy
 };
 
-static void
-lockstat_barrier_xc(void *arg0 __unused, void *arg1 __unused)
-{
-
-	membar_consumer();
-}
-
 typedef void (*dtrace_probe_func_t)(dtrace_id_t, uintptr_t, uintptr_t,
     uintptr_t, uintptr_t, uintptr_t);
 
@@ -168,9 +161,13 @@ lockstat_cas_probe(dtrace_probe_func_t old, dtrace_probe_func_t new)
 	if (lockstat_probe_func != old)
 		return false;
 
+	/* Set the probe func.  */
 	lockstat_probe_func = new;
-	membar_producer();
-	xc_wait(xc_broadcast(0, lockstat_barrier_xc, NULL, NULL));
+
+	/* Wait until all CPUs have witnessed the new probe func.  */
+	xc_barrier(0);
+
+	/* Success!  */
 	return true;
 }
 
diff --git a/sys/dev/lockstat.c b/sys/dev/lockstat.c
index b2db655caa9c..7ea34501d107 100644
--- a/sys/dev/lockstat.c
+++ b/sys/dev/lockstat.c
@@ -53,7 +53,7 @@ __KERNEL_RCSID(0, "$NetBSD: lockstat.c,v 1.26 2019/11/23 17:32:10 ad Exp $");
 #include <sys/conf.h>
 #include <sys/cpu.h>
 #include <sys/syslog.h>
-#include <sys/atomic.h>
+#include <sys/xcall.h>
 
 #include <dev/lockstat.h>
 
@@ -235,7 +235,6 @@ lockstat_start(lsenable_t *le)
 	lockstat_lockstart = le->le_lockstart;
 	lockstat_lockstart = le->le_lockstart;
 	lockstat_lockend = le->le_lockend;
-	membar_sync();
 	getnanotime(&lockstat_stime);
 	lockstat_dev_enabled = le->le_mask;
 	LOCKSTAT_ENABLED_UPDATE();
@@ -257,14 +256,10 @@ lockstat_stop(lsdisable_t *ld)
 	/* coverity[assert_side_effect] */
 	KASSERT(lockstat_dev_enabled);
 
-	/*
-	 * Set enabled false, force a write barrier, and wait for other CPUs
-	 * to exit lockstat_event().
-	 */
+	/* Disable it and wait for it to settle.  */
 	lockstat_dev_enabled = 0;
 	LOCKSTAT_ENABLED_UPDATE();
 	getnanotime(&ts);
-	tsleep(&lockstat_stop, PPAUSE, "lockstat", mstohz(10));
 
 	/*
 	 * Did we run out of buffers while tracing?
@@ -312,6 +307,20 @@ lockstat_stop(lsdisable_t *ld)
 	return error;
 }
 
+void
+lockstat_enabled_update(u_int dtrace_enabled)
+{
+
+	/*
+	 * Wait until all CPUs see the setup if we're setting it up,
+	 * kick it off, and wait until all CPUs are done using it if
+	 * we're tearing it down.
+	 */
+	xc_barrier(0);
+	lockstat_enabled = lockstat_dev_enabled | dtrace_enabled;
+	xc_barrier(0);
+}
+
 /*
  * Allocate buffers for lockstat_start().
  */
@@ -367,6 +376,9 @@ lockstat_event(uintptr_t lock, uintptr_t callsite, u_int flags, u_int count,
 	u_int event;
 	int s;
 
+	/* Lock out xc_barrier.  */
+	kpreempt_disable();
+
 #ifdef KDTRACE_HOOKS
 	uint32_t id;
 	CTASSERT((LS_NPROBES & (LS_NPROBES - 1)) == 0);
@@ -376,11 +388,11 @@ lockstat_event(uintptr_t lock, uintptr_t callsite, u_int flags, u_int count,
 #endif
 
 	if ((flags & lockstat_dev_enabled) != flags || count == 0)
-		return;
+		goto out;
 	if (lock < lockstat_lockstart || lock > lockstat_lockend)
-		return;
+		goto out;
 	if (callsite < lockstat_csstart || callsite > lockstat_csend)
-		return;
+		goto out;
 
 	callsite &= lockstat_csmask;
 	lock &= lockstat_lamask;
@@ -431,6 +443,7 @@ lockstat_event(uintptr_t lock, uintptr_t callsite, u_int flags, u_int count,
 	}
 
 	splx(s);
+out:	kpreempt_enable();
 }
 
 /*
diff --git a/sys/dev/lockstat.h b/sys/dev/lockstat.h
index 80e94f254433..e4ea6ca232c7 100644
--- a/sys/dev/lockstat.h
+++ b/sys/dev/lockstat.h
@@ -214,13 +214,10 @@ void		lockstat_probe_stub(uint32_t, uintptr_t, uintptr_t,
 #endif
 
 #if defined(_KERNEL) && NLOCKSTAT > 0
-extern volatile u_int	lockstat_enabled;
-extern volatile u_int	lockstat_dev_enabled;
-
-#define LOCKSTAT_ENABLED_UPDATE() do { \
-	lockstat_enabled = lockstat_dev_enabled | KDTRACE_LOCKSTAT_ENABLED; \
-	membar_producer(); \
-    } while (/*CONSTCOND*/0)
+extern volatile u_int lockstat_enabled;
+void lockstat_enabled_update(u_int);
+#define LOCKSTAT_ENABLED_UPDATE()					      \
+	lockstat_enabled_update(KDTRACE_LOCKSTAT_ENABLED)
 #endif
 
 #endif	/* _SYS_LOCKSTAT_H_ */