From 486320aac47a70ba38fa073d099952762d964f7b Mon Sep 17 00:00:00 2001
From: kib <kib@FreeBSD.org>
Date: Wed, 2 Mar 2016 18:46:17 +0000
Subject: [PATCH] If callout_stop_safe() noted that the callout is currently
 executing, but next invocation is cancelled while migrating,
 sleepq_check_timeout() needs to be informed that the callout is stopped. 
 Otherwise the thread switches off CPU and never become runnable, since
 running callout could have already raced with us, while the migrating and
 cancelled callout could be one which is expected to set TDP_TIMOFAIL flag for
 us.  This contradicts with the expected behaviour of callout_stop() for other
 callers, which e.g. decrement references from the callout callbacks.

Add a new flag CS_MIGRBLOCK requesting report of the situation as
'successfully stopped'.

Reviewed by:	jhb (previous version)
Tested by:	cognet, pho
PR:	200992
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D5221
---
 sys/kern/kern_timeout.c    | 10 +++++-----
 sys/kern/subr_sleepqueue.c |  3 ++-
 sys/sys/callout.h          |  8 +++++++-
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/sys/kern/kern_timeout.c b/sys/kern/kern_timeout.c
index 9c9d25f56b1d..b13a63f8c2e6 100644
--- a/sys/kern/kern_timeout.c
+++ b/sys/kern/kern_timeout.c
@@ -1155,14 +1155,14 @@ callout_schedule(struct callout *c, int to_ticks)
 }
 
 int
-_callout_stop_safe(struct callout *c, int safe, void (*drain)(void *))
+_callout_stop_safe(struct callout *c, int flags, void (*drain)(void *))
 {
 	struct callout_cpu *cc, *old_cc;
 	struct lock_class *class;
 	int direct, sq_locked, use_lock;
 	int not_on_a_list;
 
-	if (safe)
+	if ((flags & CS_DRAIN) != 0)
 		WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, c->c_lock,
 		    "calling %s", __func__);
 
@@ -1170,7 +1170,7 @@ _callout_stop_safe(struct callout *c, int safe, void (*drain)(void *))
 	 * Some old subsystems don't hold Giant while running a callout_stop(),
 	 * so just discard this check for the moment.
 	 */
-	if (!safe && c->c_lock != NULL) {
+	if ((flags & CS_DRAIN) == 0 && c->c_lock != NULL) {
 		if (c->c_lock == &Giant.lock_object)
 			use_lock = mtx_owned(&Giant);
 		else {
@@ -1253,7 +1253,7 @@ _callout_stop_safe(struct callout *c, int safe, void (*drain)(void *))
 			return (-1);
 		}
 
-		if (safe) {
+		if ((flags & CS_DRAIN) != 0) {
 			/*
 			 * The current callout is running (or just
 			 * about to run) and blocking is allowed, so
@@ -1370,7 +1370,7 @@ _callout_stop_safe(struct callout *c, int safe, void (*drain)(void *))
 				cc_exec_drain(cc, direct) = drain;
 			}
 			CC_UNLOCK(cc);
-			return (0);
+			return ((flags & CS_MIGRBLOCK) != 0);
 		}
 		CTR3(KTR_CALLOUT, "failed to stop %p func %p arg %p",
 		    c, c->c_func, c->c_arg);
diff --git a/sys/kern/subr_sleepqueue.c b/sys/kern/subr_sleepqueue.c
index bbbec920ef0d..12908f682c31 100644
--- a/sys/kern/subr_sleepqueue.c
+++ b/sys/kern/subr_sleepqueue.c
@@ -586,7 +586,8 @@ sleepq_check_timeout(void)
 	 * another CPU, so synchronize with it to avoid having it
 	 * accidentally wake up a subsequent sleep.
 	 */
-	else if (callout_stop(&td->td_slpcallout) == 0) {
+	else if (_callout_stop_safe(&td->td_slpcallout, CS_MIGRBLOCK, NULL)
+	    == 0) {
 		td->td_flags |= TDF_TIMEOUT;
 		TD_SET_SLEEPING(td);
 		mi_switch(SW_INVOL | SWT_SLEEPQTIMO, NULL);
diff --git a/sys/sys/callout.h b/sys/sys/callout.h
index 3e71c87a14ca..689b54749275 100644
--- a/sys/sys/callout.h
+++ b/sys/sys/callout.h
@@ -62,6 +62,12 @@ struct callout_handle {
 	struct callout *callout;
 };
 
+/* Flags for callout_stop_safe() */
+#define	CS_DRAIN		0x0001 /* callout_drain(), wait allowed */
+#define	CS_MIGRBLOCK		0x0002 /* Block migration, return value
+					  indicates that the callout was
+				          executing */
+
 #ifdef _KERNEL
 /* 
  * Note the flags field is actually *two* fields. The c_flags
@@ -81,7 +87,7 @@ struct callout_handle {
  */
 #define	callout_active(c)	((c)->c_flags & CALLOUT_ACTIVE)
 #define	callout_deactivate(c)	((c)->c_flags &= ~CALLOUT_ACTIVE)
-#define	callout_drain(c)	_callout_stop_safe(c, 1, NULL)
+#define	callout_drain(c)	_callout_stop_safe(c, CS_DRAIN, NULL)
 void	callout_init(struct callout *, int);
 void	_callout_init_lock(struct callout *, struct lock_object *, int);
 #define	callout_init_mtx(c, mtx, flags)					\