Re: [PATCH RESEND drm-misc-next 4/7] drm/arm/hdlcd: plane: use drm managed resources

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Hi Liviu,

On 9/30/22 18:56, Liviu Dudau wrote:
On Wed, Sep 14, 2022 at 12:03:58AM +0200, Danilo Krummrich wrote:
Do you mind trying again with my v2 (although v2 shouldn't make a difference
for this issue) and provide the back-trace when it hangs?

Hi Danilo,


I've finally got a replacement Juno board that it is stable enough.

That's great!


I've tried your v2 on top of 7860d720a84c ("drm/msm: Fix build break with recent mm tree") which
is the head of drm-next today and rmmod hangs. /proc/<pid_of_rmmod>/stack shows:

Thanks for taking the time to test the patches and providing this stacktrace.


[<0>] __synchronize_srcu.part.0+0x78/0xec
[<0>] synchronize_srcu+0xe0/0x134
[<0>] drm_dev_unplug+0x2c/0x60 [drm]
[<0>] hdlcd_drm_unbind+0x20/0xc0 [hdlcd]
[<0>] component_master_del+0xa4/0xc0
[<0>] hdlcd_remove+0x1c/0x2c [hdlcd]
[<0>] platform_remove+0x28/0x60
[<0>] device_remove+0x4c/0x80
[<0>] device_release_driver_internal+0x1e4/0x250
[<0>] driver_detach+0x50/0xe0
[<0>] bus_remove_driver+0x5c/0xbc
[<0>] driver_unregister+0x30/0x60
[<0>] platform_driver_unregister+0x14/0x20
[<0>] hdlcd_platform_driver_exit+0x1c/0xe40 [hdlcd]
[<0>] __arm64_sys_delete_module+0x18c/0x240
[<0>] invoke_syscall+0x48/0x114
[<0>] el0_svc_common.constprop.0+0xcc/0xec
[<0>] do_el0_svc+0x2c/0xc0
[<0>] el0_svc+0x2c/0x84
[<0>] el0t_64_sync_handler+0x11c/0x150
[<0>] el0t_64_sync+0x18c/0x190


I think I figured it out. I messed up two of the srcu read-side critical sections by overlooking alternate return paths within those sections - yikes!

I also found a potential use-after-free I accidentally introduced while removing the patch in v2.

Finally, I added a patch to remove unnecessary calls to drm_mode_config_cleanup() and replaced drm_mode_config_init() with drmm_mode_config_init().

I will send you a v3 containing those fixes.

My quick guess would be that the mixing of managed and unmanaged APIs manages to
confuse the sleepable RCUs and we get the hang.

I guess you're referring to drm_crtc_init_with_planes() and providing a .destroy callback in hdlcd_crtc_funcs. Actually, this .destroy callback is handled in the same way as when initializing the crtc with drmm_crtc_init_with_planes(), hence I don't think we have a mix of managed and unmanaged APIs here.

drmm_crtc_init_with_planes() just calls __drm_crtc_init_with_planes() and adds the cleanup via drmm_add_action_or_reset().

Having a .destroy callback registered via drm_crtc_init_with_planes() ends up the same way, since drm_mode_config_init() simply calls drmm_mode_config_init(), which also registers drm_mode_config_cleanup() (ultimately calling the crtc->destroy callback) via drmm_add_action_or_reset().

- Danilo

Will chat with Daniel Vetter next
week at XDC on what would be the best approach here.

Best regards,
Liviu




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