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@xxxxxxx>
Reported-by: Yudong Wang <yudong.wang@xxxxxxxxx>
Tested-by: Yudong Wang <yudong.wang@xxxxxxxxx>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>
---
v1 -> v2:
- No function changes.
- Add 'Acked-by' from Thomas Zimmermann.
[ Thanks Thomas Zimmermann for reviewing this patch. ]
- Add 'Tested-by' from Yudong Wang.
- Add comments to drm_fb_helper_damage() about the early return on oops_in_progress.
drivers/gpu/drm/drm_fb_helper.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d612133e2cf7..6ad75034c966 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -628,6 +628,17 @@ 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);
--
2.17.1
