Claude review: drm/vkms: Fix ABBA deadlock in vblank disable and timer callback

[Claude Code Review Bot <claude-review@example.com>]

Overall Series Review

Subject: drm/vkms: Fix ABBA deadlock in vblank disable and timer callback
Author: <w15303746062@163.com>
Patches: 2
Reviewed: 2026-05-16T09:05:03.765320

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This is a single patch targeting stable tree 6.18.y that attempts to fix an ABBA deadlock in the VKMS driver between `drm_vblank_disable_and_save()` (which holds `vblank_time_lock` and calls `hrtimer_cancel()`) and `vkms_vblank_simulate()` (an hrtimer callback that tries to acquire `vblank_time_lock` via `drm_handle_vblank()`).

**The deadlock analysis is correct** — this is a real lock ordering issue. In fact, **mainline has already fixed this exact class of bug** as part of the `DRM_CRTC_VBLANK_TIMER_FUNCS` refactoring. The new `drm_crtc_vblank_cancel_timer()` function at `drm_vblank.c:2261` uses the same `hrtimer_try_to_cancel()` approach plus interval-clearing, and the comment at line 2268-2271 explicitly documents this deadlock:

> Calling hrtimer_cancel() can result in a deadlock with DRM's vblank_time_lock and hrtimers' softirq_expiry_lock.

However, this patch has **significant issues** that make it unsuitable for merging as-is.

**Verdict: Needs revision.** The fix idea is sound but the implementation is incomplete and the patch targets the wrong tree.

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Generated by Claude Code Patch Reviewer