Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!cbosgd!gatech!robin
From: robin@gatech.CSNET (Robin Cutshaw)
Newsgroups: net.bugs.4bsd,net.unix-wizards
Subject: flock bug report with fix
Message-ID: <2767@gatech.CSNET>
Date: Thu, 20-Feb-86 14:58:49 EST
Article-I.D.: gatech.2767
Posted: Thu Feb 20 14:58:49 1986
Date-Received: Fri, 21-Feb-86 06:17:34 EST
Organization: School of Information and Computer Science, Georgia Tech, Atlanta
Lines: 111
Keywords: flock
Xref: watmath net.bugs.4bsd:1966 net.unix-wizards:16864



Program: /sys/sys/sys_inode.c
Severity: major
Releases:  BSD4.2, BRL 3.0, Ultrix 1.1

Description:
  The flock system call invokes ino_lock() in /sys/sys/sys_inode.c.
Ino_lock() loops waiting for release of an exclusive lock and then
loops waiting for release of shared locks if the request is an
exclusive lock.  When all shared locks release, any AND ALL exclusive
lock requests will be granted (yes more than one exclusive lock will
pass).

Repeat-By:
  Crank up a process that will request a shared lock and wait for a
few seconds, then release.  After the shared lock process has locked
but before it releases, crank up multiple processes requesting
exclusive locks.  When the shared lock clears, all of the exclusive
requests will succeed simultaneously.

Fix:
  In vanilla 4.2 :

*** sys_inode.c.4.2	Fri Jul 29 10:07:21 1983
--- sys_inode.c.4.2.rkc	Wed Feb 19 12:08:35 1986
***************
*** 390,395
  		/*
  		 * Must wait for any shared locks to finish
  		 * before we try to apply a exclusive lock.
  		 */
  		while (ip->i_flag & ISHLOCK) {
  			if (cmd & LOCK_NB)

--- 390,398 -----
  		/*
  		 * Must wait for any shared locks to finish
  		 * before we try to apply a exclusive lock.
+                  *
+                  * We must also wait if an exclusive lock
+                  * was applied during our last wait. (gatech!robin)
  		 */
  		while (ip->i_flag & ISHLOCK) {
  			if (cmd & LOCK_NB)
***************
*** 404,409
  			}
  			ip->i_flag |= ILWAIT;
  			sleep((caddr_t)&ip->i_shlockc, PLOCK);
  		}
  	}
  	if (fp->f_flag & (FSHLOCK|FEXLOCK))

--- 407,414 -----
  			}
  			ip->i_flag |= ILWAIT;
  			sleep((caddr_t)&ip->i_shlockc, PLOCK);
+ 			if (ip->i_flag & IEXLOCK)
+ 				goto again;
  		}
  	}
  	if (fp->f_flag & (FSHLOCK|FEXLOCK))
====================================================================
  In BRL unix :

*** sys_inode.c.brl	Thu May  2 16:47:31 1985
--- sys_inode.c.brl.rkc	Wed Feb 19 10:56:14 1986
***************
*** 400,405
  		/*
  		 * Must wait for any shared locks to finish
  		 * before we try to apply a exclusive lock.
  		 */
  		while (ip->i_flag & ISHLOCK) {
  			/*

--- 400,408 -----
  		/*
  		 * Must wait for any shared locks to finish
  		 * before we try to apply a exclusive lock.
+                  *
+                  * We must also wait if an exclusive lock
+                  * was applied during our last wait. (gatech!robin)
  		 */
  		while (ip->i_flag & ISHLOCK) {
  			/*
***************
*** 414,419
  				return (EWOULDBLOCK);
  			ip->i_flag |= ILWAIT;
  			csleep((caddr_t)&ip->i_shlockc, PLOCK, "EXCL OPEN");
  		}
  	}
  	if (fp->f_flag & (FSHLOCK|FEXLOCK))

--- 417,424 -----
  				return (EWOULDBLOCK);
  			ip->i_flag |= ILWAIT;
  			csleep((caddr_t)&ip->i_shlockc, PLOCK, "EXCL OPEN");
+ 			if (ip->i_flag & IEXLOCK)
+ 				goto again;
  		}
  	}
  	if (fp->f_flag & (FSHLOCK|FEXLOCK))
====================================================================
-- 
Robin K. Cutshaw
School of Information & Computer Science, Georgia Tech, Atlanta GA 30332
CSNet:  robin @ GATech		ARPA:  robin%GATech.CSNet @ CSNet-Relay.ARPA
uucp:  ...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!robin