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

[Rob Clark <robdclark@gmail.com>]

On Mon, Jan 25, 2021 at 3:49 PM Stephen Boyd <swboyd@chromium.org> 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

nit, s/on/one/ ?

BR,
-R

> 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>
> ---
>  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
>