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Date: Wed, 5 Feb 1997 16:29:28 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199702050529.QAA21597@godzilla.zeta.org.au>
To: bugs@freebsd.org
Subject: ipl not preserved by cpu_switch()
Cc: davidg@freebsd.org, peter@freebsd.org
Sender: owner-bugs@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk

cpu_switch() doesn't preserve the ipl.  This seems to be fairly harmless.
It only makes a difference for some cases involving stopped processes.
One case is easy to duplicate: run something under gdb, stop it with ^C,
and then quit gdb.  The stopped process usually exits immediately and
calls exit1() at splhigh().

Bruce

diff -c2 kern_synch.c~ kern_synch.c
*** kern_synch.c~	Wed Jan 15 05:04:32 1997
--- kern_synch.c	Tue Feb  4 11:30:13 1997
***************
*** 528,533 ****
--- 527,552 ----
  	register struct rlimit *rlim;
  	register long s, u;
+ 	int x;
  	struct timeval tv;
  
+ 	/*
+ 	 * XXX this spl is almost unnecessary.  It is partly to allow for
+ 	 * sloppy callers that don't do it (issignal() via CURSIG() is the
+ 	 * main offender).  It is partly to work around a bug in the i386
+ 	 * cpu_switch() (the ipl is not preserved).  We ran for years
+ 	 * without it.  I think there was only a interrupt latency problem.
+ 	 * The main caller, tlseep(), does an splx() a couple of instructions
+ 	 * after calling here.  The buggy caller, issignal(), usually calls
+ 	 * here at spl0() and sometimes returns at splhigh().  The process
+ 	 * then runs for a little too long at splhigh().  The ipl gets fixed
+ 	 * when the process returns to user mode (or earlier).
+ 	 *
+ 	 * It would probably be better to always call here at spl0(). Callers
+ 	 * are prepared to give up control to another process, so they must
+ 	 * be prepared to be interrupted.  The clock stuff here may not
+ 	 * actually need splstatclock().
+ 	 */
+ 	x = splstatclock();
+ 
  #ifdef DEBUG
  	if (p->p_simple_locks)
***************
*** 574,577 ****
--- 593,597 ----
  	cpu_switch(p);
  	microtime(&runtime);
+ 	splx(x);
  }