drm/fb-helper: Don't schedule_work() to flush frame buffer during panic()

Sometimes the system [1] hangs on x86 I/O machine checks. However, the
expected behavior is to reboot the system, as the machine check handler
ultimately triggers a panic(), initiating a reboot in the last step.

The root cause is that sometimes the panic() is blocked when
drm_fb_helper_damage() invoking schedule_work() to flush the frame buffer.
This occurs during the process of flushing all messages to the frame
buffer driver as shown in the following call trace:

  Machine check occurs [2]:
    panic()
      console_flush_on_panic()
        console_flush_all()
          console_emit_next_record()
            con->write()
              vt_console_print()
                hide_cursor()
                  vc->vc_sw->con_cursor()
                    fbcon_cursor()
                      ops->cursor()
                        bit_cursor()
                          soft_cursor()
                            info->fbops->fb_imageblit()
                              drm_fbdev_generic_defio_imageblit()
                                drm_fb_helper_damage_area()
                                  drm_fb_helper_damage()
                                    schedule_work() // <--- blocked here
    ...
    emergency_restart()  // wasn't invoked, so no reboot.

During panic(), except the panic CPU, all the other CPUs are stopped.
In schedule_work(), the panic CPU requires the lock of worker_pool to
queue the work on that pool, while the lock may have been token by some
other stopped CPU. So schedule_work() is blocked.

Additionally, during a panic(), since there is no opportunity to execute
any scheduled work, it's safe to fix this issue by skipping schedule_work()
on 'oops_in_progress' in drm_fb_helper_damage().

[1] Enable the kernel option CONFIG_FRAMEBUFFER_CONSOLE,
    CONFIG_DRM_FBDEV_EMULATION, and boot with the 'console=tty0'
    kernel command line parameter.

[2] Set 'panic_timeout' to a non-zero value before calling panic().
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Yudong Wang <yudong.wang@intel.com>
Tested-by: Yudong Wang <yudong.wang@intel.com>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240703141737.75378-1-qiuxu.zhuo@intel.comSigned-off-by: Maarten Lankhorst,,, <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -631,6 +631,17 @@ static void drm_fb_helper_add_damage_clip(struct drm_fb_helper *helper, u32 x, u
 static void drm_fb_helper_damage(struct drm_fb_helper *helper, u32 x, u32 y,
 				 u32 width, u32 height)
 {
+	/*
+	 * This function may be invoked by panic() to flush the frame
+	 * buffer, where all CPUs except the panic CPU are stopped.
+	 * During the following schedule_work(), the panic CPU needs
+	 * the worker_pool lock, which might be held by a stopped CPU,
+	 * causing schedule_work() and panic() to block. Return early on
+	 * oops_in_progress to prevent this blocking.
+	 */
+	if (oops_in_progress)
+		return;
+
 	drm_fb_helper_add_damage_clip(helper, x, y, width, height);
 
 	schedule_work(&helper->damage_work);