svn commit: r319720 - head/sys/dev/vt

[Jonathan T. Looney]

Author: jtl
Date: Thu Jun  8 20:47:18 2017
New Revision: 319720
URL: https://svnweb.freebsd.org/changeset/base/319720

Log:
  With EARLY_AP_STARTUP enabled, we are seeing crashes in softclock_call_cc()
  during bootup. Debugging information shows that softclock_call_cc() is
  trying to execute the vt_consdev.vd_timer callout, and the callout
  structure contains a NULL c_func.
  
  This appears to be due to a race between vt_upgrade() running
  callout_reset() and vt_resume_flush_timer() calling callout_schedule().
  
  Fix the race by ensuring that vd_timer_armed is always set before
  attempting to (re)schedule the callout.
  
  Discussed with:	emaste
  MFC after:	2 weeks
  Sponsored by:	Netflix
  Differential Revision:	https://reviews.freebsd.org/D9828

Modified:
  head/sys/dev/vt/vt_core.c

Modified: head/sys/dev/vt/vt_core.c
==============================================================================
--- head/sys/dev/vt/vt_core.c	Thu Jun  8 20:41:28 2017	(r319719)
+++ head/sys/dev/vt/vt_core.c	Thu Jun  8 20:47:18 2017	(r319720)
@@ -2610,10 +2610,17 @@ vt_upgrade(struct vt_device *vd)
 		/* Init 25 Hz timer. */
 		callout_init_mtx(&vd->vd_timer, &vd->vd_lock, 0);
 
-		/* Start timer when everything ready. */
+		/*
+		 * Start timer when everything ready.
+		 * Note that the operations here are purposefully ordered.
+		 * We need to ensure vd_timer_armed is non-zero before we set
+		 * the VDF_ASYNC flag. That prevents this function from
+		 * racing with vt_resume_flush_timer() to update the
+		 * callout structure.
+		 */
+		atomic_add_acq_int(&vd->vd_timer_armed, 1);
 		vd->vd_flags |= VDF_ASYNC;
 		callout_reset(&vd->vd_timer, hz / VT_TIMERFREQ, vt_timer, vd);
-		vd->vd_timer_armed = 1;
 		register_handlers = 1;
 	}