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From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Thu, 30 Oct 2014 05:21:13 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r273843 - head/sys/kern
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Author: mjg
Date: Thu Oct 30 05:21:12 2014
New Revision: 273843
URL: https://svnweb.freebsd.org/changeset/base/273843

Log:
  filedesc: microoptimize fget_unlocked by retrying obtaining reference count
  without restarting whole lookup
  
  Restart is only needed when fp was closed by current process, which is a much
  rarer event than ref/deref by some other thread.

Modified:
  head/sys/kern/kern_descrip.c

Modified: head/sys/kern/kern_descrip.c
==============================================================================
--- head/sys/kern/kern_descrip.c	Thu Oct 30 05:10:33 2014	(r273842)
+++ head/sys/kern/kern_descrip.c	Thu Oct 30 05:21:12 2014	(r273843)
@@ -2359,6 +2359,7 @@ fget_unlocked(struct filedesc *fdp, int 
 			}
 		}
 #endif
+	retry:
 		count = fp->f_count;
 		if (count == 0) {
 			fdt = fdp->fd_files;
@@ -2368,10 +2369,8 @@ fget_unlocked(struct filedesc *fdp, int 
 		 * Use an acquire barrier to force re-reading of fdt so it is
 		 * refreshed for verification.
 		 */
-		if (atomic_cmpset_acq_int(&fp->f_count, count, count + 1) == 0) {
-			fdt = fdp->fd_files;
-			continue;
-		}
+		if (atomic_cmpset_acq_int(&fp->f_count, count, count + 1) == 0)
+			goto retry;
 		fdt = fdp->fd_files;
 #ifdef	CAPABILITIES
 		if (seq_consistent_nomb(fd_seq(fdt, fd), seq))