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From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Wed, 10 Aug 2016 15:25:45 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r303922 - head/sys/kern
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Author: mjg
Date: Wed Aug 10 15:25:44 2016
New Revision: 303922
URL: https://svnweb.freebsd.org/changeset/base/303922

Log:
  ktrace: do a lockless check on fork to see if tracing is enabled
  
  This saves 2 lock acquisitions in the common case.

Modified:
  head/sys/kern/kern_ktrace.c

Modified: head/sys/kern/kern_ktrace.c
==============================================================================
--- head/sys/kern/kern_ktrace.c	Wed Aug 10 15:24:15 2016	(r303921)
+++ head/sys/kern/kern_ktrace.c	Wed Aug 10 15:25:44 2016	(r303922)
@@ -572,9 +572,14 @@ void
 ktrprocfork(struct proc *p1, struct proc *p2)
 {
 
+	MPASS(p2->p_tracevp == NULL);
+	MPASS(p2->p_traceflag == 0);
+
+	if (p1->p_traceflag == 0)
+		return;
+
 	PROC_LOCK(p1);
 	mtx_lock(&ktrace_mtx);
-	KASSERT(p2->p_tracevp == NULL, ("new process has a ktrace vnode"));
 	if (p1->p_traceflag & KTRFAC_INHERIT) {
 		p2->p_traceflag = p1->p_traceflag;
 		if ((p2->p_tracevp = p1->p_tracevp) != NULL) {