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From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Sun, 22 Jun 2014 21:37:28 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r267755 - head/sys/kern
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Author: mjg
Date: Sun Jun 22 21:37:27 2014
New Revision: 267755
URL: http://svnweb.freebsd.org/changeset/base/267755

Log:
  Don't take filedesc lock in fdunshare().
  
  We can read refcnt safely and only care if it is equal to 1.
  
  If it could suddenly change from 1 to something bigger the code would be
  buggy even in the previous form and transitions from > 1 to 1 are equally racy
  and harmless (we copy even though there is no need).
  
  MFC after:	1 week

Modified:
  head/sys/kern/kern_descrip.c

Modified: head/sys/kern/kern_descrip.c
==============================================================================
--- head/sys/kern/kern_descrip.c	Sun Jun 22 21:28:05 2014	(r267754)
+++ head/sys/kern/kern_descrip.c	Sun Jun 22 21:37:27 2014	(r267755)
@@ -1852,17 +1852,14 @@ fdshare(struct filedesc *fdp)
 void
 fdunshare(struct proc *p, struct thread *td)
 {
+	struct filedesc *tmp;
 
-	FILEDESC_XLOCK(p->p_fd);
-	if (p->p_fd->fd_refcnt > 1) {
-		struct filedesc *tmp;
-
-		FILEDESC_XUNLOCK(p->p_fd);
-		tmp = fdcopy(p->p_fd);
-		fdescfree(td);
-		p->p_fd = tmp;
-	} else
-		FILEDESC_XUNLOCK(p->p_fd);
+	if (p->p_fd->fd_refcnt == 1)
+		return;
+
+	tmp = fdcopy(p->p_fd);
+	fdescfree(td);
+	p->p_fd = tmp;
 }
 
 /*