[PATCHv2] drm/msm/kms: Make a lock_class_key for each crtc mutex

[PATCHv2] drm/msm/kms: Make a lock_class_key for each crtc mutex

[Stephen Boyd]

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 one 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>
---

Changes from v1:
 * Fixed typo in commit text

 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

Re: [PATCHv2] drm/msm/kms: Make a lock_class_key for each crtc mutex

[patchwork-bot+linux-arm-msm]

Hello:

This patch was applied to qcom/linux.git (refs/heads/for-next):

On Thu, 28 Jan 2021 09:09:29 -0800 you 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
> 
> [...]

Here is the summary with links:
  - [PATCHv2] drm/msm/kms: Make a lock_class_key for each crtc mutex
    https://git.kernel.org/qcom/c/6ec935180961

You are awesome, thank you!
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