Re: [PATCH] drm/msm/kms: Make a lock_class_key for each crtc mutex

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 25, 2021 at 03:49:01PM -0800, Stephen Boyd wrote:
> Lockdep complains about an AA deadlock when rebooting the device.
> 
> ============================================
> WARNING: possible recursive locking detected
> 5.4.91 #1 Not tainted
> --------------------------------------------
> reboot/5213 is trying to acquire lock:
> ffffff80d13391b0 (&kms->commit_lock[i]){+.+.}, at: lock_crtcs+0x60/0xa4
> 
> but task is already holding lock:
> ffffff80d1339110 (&kms->commit_lock[i]){+.+.}, at: lock_crtcs+0x60/0xa4
> 
> other info that might help us debug this:
> Possible unsafe locking scenario:
> 
> CPU0
> ----
> lock(&kms->commit_lock[i]);
> lock(&kms->commit_lock[i]);
> 
> *** DEADLOCK ***
> 
> May be due to missing lock nesting notation
> 
> 6 locks held by reboot/5213:
> __arm64_sys_reboot+0x148/0x2a0
> device_shutdown+0x10c/0x2c4
> drm_atomic_helper_shutdown+0x48/0xfc
> modeset_lock+0x120/0x24c
> lock_crtcs+0x60/0xa4
> 
> stack backtrace:
> CPU: 4 PID: 5213 Comm: reboot Not tainted 5.4.91 #1
> Hardware name: Google Pompom (rev1) with LTE (DT)
> Call trace:
> dump_backtrace+0x0/0x1dc
> show_stack+0x24/0x30
> dump_stack+0xfc/0x1a8
> __lock_acquire+0xcd0/0x22b8
> lock_acquire+0x1ec/0x240
> __mutex_lock_common+0xe0/0xc84
> mutex_lock_nested+0x48/0x58
> lock_crtcs+0x60/0xa4
> msm_atomic_commit_tail+0x348/0x570
> commit_tail+0xdc/0x178
> drm_atomic_helper_commit+0x160/0x168
> drm_atomic_commit+0x68/0x80
> 
> This is because lockdep thinks all the locks taken in lock_crtcs() are
> the same lock, when they actually aren't. That's because we call
> mutex_init() in msm_kms_init() and that assigns on static key for every
> lock initialized in this loop. Let's allocate a dynamic number of
> lock_class_keys and assign them to each lock so that lockdep can figure
> out an AA deadlock isn't possible here.
> 
> Fixes: b3d91800d9ac ("drm/msm: Fix race condition in msm driver with async layer updates")
> Cc: Krishna Manikandan <mkrishn@codeaurora.org>
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>

This smells like throwing more bad after initial bad code ...

First a rant: https://blog.ffwll.ch/2020/08/lockdep-false-positives.html

Yes I know the locking you're doing here is correct, but that goes to the
second issue: Why is this needed? atomic_async_update helpers are supposed
to take care of ordering fun like this, if they're not, we need to address
things there. The problem that

commit b3d91800d9ac35014e0349292273a6fa7938d402
Author: Krishna Manikandan <mkrishn@codeaurora.org>
Date:   Fri Oct 16 19:40:43 2020 +0530

    drm/msm: Fix race condition in msm driver with async layer updates

is _the_ reason we have drm_crtc_commit to track stuff, and Maxime has
recently rolled out a pile of changes to vc4 to use these things
correctly. Hacking some glorious hand-rolled locking for synchronization
of updates really should be the exception for kms drivers, not the rule.
And this one here doesn't look like an exception by far (the one legit I
know of is the locking issues amdgpu has between atomic_commit_tail and
gpu reset, and that one is really nasty, so not going to get fixed in
helpers, ever).

Cheers, Daniel

> ---
>  drivers/gpu/drm/msm/msm_kms.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_kms.h b/drivers/gpu/drm/msm/msm_kms.h
> index d8151a89e163..4735251a394d 100644
> --- a/drivers/gpu/drm/msm/msm_kms.h
> +++ b/drivers/gpu/drm/msm/msm_kms.h
> @@ -157,6 +157,7 @@ struct msm_kms {
>  	 * from the crtc's pending_timer close to end of the frame:
>  	 */
>  	struct mutex commit_lock[MAX_CRTCS];
> +	struct lock_class_key commit_lock_keys[MAX_CRTCS];
>  	unsigned pending_crtc_mask;
>  	struct msm_pending_timer pending_timers[MAX_CRTCS];
>  };
> @@ -166,8 +167,11 @@ static inline int msm_kms_init(struct msm_kms *kms,
>  {
>  	unsigned i, ret;
>  
> -	for (i = 0; i < ARRAY_SIZE(kms->commit_lock); i++)
> -		mutex_init(&kms->commit_lock[i]);
> +	for (i = 0; i < ARRAY_SIZE(kms->commit_lock); i++) {
> +		lockdep_register_key(&kms->commit_lock_keys[i]);
> +		__mutex_init(&kms->commit_lock[i], "&kms->commit_lock[i]",
> +			     &kms->commit_lock_keys[i]);
> +	}
>  
>  	kms->funcs = funcs;
>  
> 
> base-commit: 19c329f6808995b142b3966301f217c831e7cf31
> -- 
> https://chromeos.dev
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch