Change callout use counter to use C11 atomics.

In order to get some coverage of C11 atomics in kernelspace, switch at
least one piece of code in kernelspace to use C11 atomics instead of
<machine/atomic.h>.

While there, slightly improve the code by adding an assertion to prevent
the use count from going negative.

1 files changed, 15 insertions, 10 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 362f5eed4449..cd8b232f83f1 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -47,6 +47,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/fcntl.h>
 #include <sys/kthread.h>
 #include <sys/selinfo.h>
+#include <sys/stdatomic.h>
 #include <sys/queue.h>
 #include <sys/event.h>
 #include <sys/eventvar.h>
@@ -182,9 +183,9 @@ static struct filterops user_filtops = {
 };
 
 static uma_zone_t	knote_zone;
-static int 		kq_ncallouts = 0;
-static int 		kq_calloutmax = (4 * 1024);
-SYSCTL_INT(_kern, OID_AUTO, kq_calloutmax, CTLFLAG_RW,
+static atomic_uint	kq_ncallouts = ATOMIC_VAR_INIT(0);
+static unsigned int 	kq_calloutmax = 4 * 1024;
+SYSCTL_UINT(_kern, OID_AUTO, kq_calloutmax, CTLFLAG_RW,
     &kq_calloutmax, 0, "Maximum number of callouts allocated for kqueue");
 
 /* XXX - ensure not KN_INFLUX?? */
@@ -549,13 +550,15 @@ static int
 filt_timerattach(struct knote *kn)
 {
 	struct callout *calloutp;
+	unsigned int ncallouts;
 
-	atomic_add_int(&kq_ncallouts, 1);
-
-	if (kq_ncallouts >= kq_calloutmax) {
-		atomic_add_int(&kq_ncallouts, -1);
-		return (ENOMEM);
-	}
+	ncallouts = atomic_load_explicit(&kq_ncallouts, memory_order_relaxed);
+	do {
+		if (ncallouts >= kq_calloutmax)
+			return (ENOMEM);
+	} while (!atomic_compare_exchange_weak_explicit(&kq_ncallouts,
+	    &ncallouts, ncallouts + 1, memory_order_relaxed,
+	    memory_order_relaxed));
 
 	kn->kn_flags |= EV_CLEAR;		/* automatically set */
 	kn->kn_status &= ~KN_DETACHED;		/* knlist_add usually sets it */
@@ -573,11 +576,13 @@ static void
 filt_timerdetach(struct knote *kn)
 {
 	struct callout *calloutp;
+	unsigned int old;
 
 	calloutp = (struct callout *)kn->kn_hook;
 	callout_drain(calloutp);
 	free(calloutp, M_KQUEUE);
-	atomic_add_int(&kq_ncallouts, -1);
+	old = atomic_fetch_sub_explicit(&kq_ncallouts, 1, memory_order_relaxed);
+	KASSERT(old > 0, ("Number of callouts cannot become negative"));
 	kn->kn_status |= KN_DETACHED;	/* knlist_remove usually clears it */
 }