From: jsg <jsg@openbsd.org>
Date: Tue, 9 Jan 2018 13:48:36 +0000 (+0000)
Subject: Work around a problem with ww_mutexes in the drm modeset lock.
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=564211fc8324e1aa1b07f1b6aef22d3c83ada672;p=openbsd

Work around a problem with ww_mutexes in the drm modeset lock.
Screen blanks often result in a NULL dereference in __ww_mutex_lock with
lock->acquired being non zero and lock->ctx NULL.  mpi@ also reported
it occuring when switching from Xorg to a virtual terminal.

ok mpi@
---

diff --git a/sys/dev/pci/drm/linux_ww_mutex.h b/sys/dev/pci/drm/linux_ww_mutex.h
index 69d50d4c669..fb1b26791f6 100644
--- a/sys/dev/pci/drm/linux_ww_mutex.h
+++ b/sys/dev/pci/drm/linux_ww_mutex.h
@@ -163,7 +163,8 @@ __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx, bool slow, bo
                          *   the `younger` process gives up all it's
                          *   resources.
 			 */
-			if (slow || ctx == NULL || ctx->stamp < lock->ctx->stamp) {
+			if (slow || ctx == NULL ||
+			    (lock->ctx != NULL && ctx->stamp < lock->ctx->stamp)) {
 				int s = msleep(lock, &lock->lock,
 					       intr ? PCATCH : 0,
 					       ctx ? ctx->ww_class->name : "ww_mutex_lock", 0);