linux-kernel - [PATCH] drm/vblank: Fix hard lockup in drm_handle

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Message-ID: <20250510094757.4174662-1-zengheng4@huawei.com>
Date: Sat, 10 May 2025 17:47:57 +0800
From: Zeng Heng <zengheng4@...wei.com>
To: <tzimmermann@...e.de>, <airlied@...il.com>,
	<maarten.lankhorst@...ux.intel.com>, <mario.kleiner@...bingen.mpg.de>,
	<airlied@...hat.com>, <mripard@...nel.org>, <simona@...ll.ch>
CC: <bobo.shaobowang@...wei.com>, <dri-devel@...ts.freedesktop.org>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH] drm/vblank: Fix hard lockup in drm_handle_vblank()

When we performed fuzz testing on DRM using syzkaller, we encountered
the following hard lockup issue:

Kernel panic - not syncing: Hard LOCKUP
CPU: 3 PID: 0 Comm: swapper/3 Not tainted 6.6.0+ #21
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
Call Trace:
 <IRQ>
 hrtimer_cancel+0x52/0x70 kernel/time/hrtimer.c:1449
 __disable_vblank drivers/gpu/drm/drm_vblank.c:434 [inline]
 drm_vblank_disable_and_save+0x27f/0x3c0 drivers/gpu/drm/drm_vblank.c:478
 vblank_disable_fn+0x15d/0x1b0 drivers/gpu/drm/drm_vblank.c:495
 call_timer_fn+0x39/0x280 kernel/time/timer.c:1700
 expire_timers+0x22d/0x3c0 kernel/time/timer.c:1751
 __run_timers kernel/time/timer.c:2022 [inline]
 run_timer_softirq+0x315/0x8a0 kernel/time/timer.c:2035
 handle_softirqs+0x195/0x580 kernel/softirq.c:553
 __do_softirq kernel/softirq.c:587 [inline]
 </IRQ>

This is a deadlock issue as follows:

    CPU3				CPU 7

vblank_disable_fn()
  drm_vblank_disable_and_save()
  spin_lock(vblank_time_lock)
				hrtimer_interrupt()
				  vkms_vblank_simulate()
				    drm_handle_vblank()
				      // wait for CPU3 to release vblank_time_lock
				      spin_lock(vblank_time_lock)
    vkms_disable_vblank()
      // wait for vblank_hrtimer on CPU7 to finish
      hrtimer_cancel(vblank_hrtimer)

The call of hrtimer_cancel() has hold vblank_time_lock which would prevent
completion of the hrtimer's callback function.

Therefore, in drm_handle_vblank(), we move the check for the
vblank->enabled variable to the time when vblank_time_lock() is acquired.
If the CRTC event has already been canceled, the drm_handle_vblank() will
be terminated and will no longer attempt to acquire vblank_time_lock.

In the same time, in drm_vblank_disable_and_save(), we set vblank->enabled
to disable before calling hrtimer_cancel() to avoid endless waiting in
hrtimer_cancel_wait_running().

Fixes: 27641c3f003e ("drm/vblank: Add support for precise vblank timestamping.")
Signed-off-by: Zeng Heng <zengheng4@...wei.com>
---
 drivers/gpu/drm/drm_vblank.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 78958ddf8485..56b80e5ede2a 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -471,6 +471,8 @@ void drm_vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
 	if (!vblank->enabled)
 		goto out;
 
+	vblank->enabled = false;
+
 	/*
 	 * Update the count and timestamp to maintain the
 	 * appearance that the counter has been ticking all along until
@@ -479,7 +481,6 @@ void drm_vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
 	 */
 	drm_update_vblank_count(dev, pipe, false);
 	__disable_vblank(dev, pipe);
-	vblank->enabled = false;
 
 out:
 	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
@@ -1932,14 +1933,13 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned int pipe)
 	 * vblank enable/disable, as this would cause inconsistent
 	 * or corrupted timestamps and vblank counts.
 	 */
-	spin_lock(&dev->vblank_time_lock);
-
-	/* Vblank irq handling disabled. Nothing to do. */
-	if (!vblank->enabled) {
-		spin_unlock(&dev->vblank_time_lock);
-		spin_unlock_irqrestore(&dev->event_lock, irqflags);
-		return false;
-	}
+	do {
+		/* Vblank irq handling disabled. Nothing to do. */
+		if (!vblank->enabled) {
+			spin_unlock_irqrestore(&dev->event_lock, irqflags);
+			return false;
+		}
+	} while (!spin_trylock(&dev->vblank_time_lock));
 
 	drm_update_vblank_count(dev, pipe, true);
 
-- 
2.25.1