From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 535FD607826F for ; Mon, 2 Nov 2026 14:53:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=FMipYyc0 Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id C3D0A10EF66; Mon, 2 Nov 2026 14:53:05 +0000 (UTC) Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by gabe.freedesktop.org (Postfix) with ESMTPS id DB4D810EF64 for ; Mon, 2 Nov 2026 14:52:57 +0000 (UTC) Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 5F46A605D4; Mon, 2 Nov 2026 14:52:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EAD191F00A16; Mon, 2 Nov 2026 14:52:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793631177; bh=/vmAi3qz/MUcn21z2Zel3BJOCM1kWxGyScvi4/JSGQM=; h=From:Subject:Reply-To:To:Cc:In-Reply-To:References:Date; b=FMipYyc0pX/ecZmgopAhQpthQPlntfnm3bQCEwOs9L6xt5Oxid8+fIUU1RhnVkoE3 kz1HbVkY068jKBptDtnoJ04IAPK7K7slpJWEkjTlI9HXhQwTGqw40zgwUtvj8nXhsW bcD1wU+jCKPZ320mUY2wr0pNIEcdP3SPfrHholMsUeYbQjuLLFHw0TEd7rczqfTtLZ VuziWA+VJWIggKWm0puIG3My40wUM/9ExV7Y/hrj/ZafvChvN+uN+s4RSuOrBVSLBi B7hmIOdPpVYYkrtZSkrwuQUdZ4iYM4jB47aF+I9qXa/CH/4BPpx7l4bwvdGa9dBybi 6JYZD/fPh1imA== Message-ID: From: sashiko-bot@kernel.org Subject: Re: [PATCH] drm/vblank: Restart enabling vblank timer on possible deadlock To: "Thomas Zimmermann" Cc: dri-devel@lists.freedesktop.org In-Reply-To: <20261102143919.1672293-1-tzimmermann@suse.de> References: <20261102143919.1672293-1-tzimmermann@suse.de> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Date: Mon, 02 Nov 2026 14:52:56 +0000 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: sashiko-reviews@lists.linux.dev Sender: "dri-devel" X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 X-Archived-By: Thank you for your contribution! Sashiko AI review found 1 potential issue(= s) to consider: - [High] drm/vblank: deadlock in drm_vblank_enable() retry when outer locks= are held -- commit 0a0f76a052c65f5a2ede08f67964f40f4ca7f5fc Author: Thomas Zimmermann drm/vblank: Restart enabling vblank timer on possible deadlock This commit updates drm_crtc_vblank_start_timer() to return -EDEADLK when hrtimer_try_to_cancel() indicates the vblank hrtimer callback is currently running, and updates drm_vblank_enable() to drop and reacquire vblank_time_lock before retrying. > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c > index f90fb2d13e423..5a4c11e57484c 100644 > --- a/drivers/gpu/drm/drm_vblank.c > +++ b/drivers/gpu/drm/drm_vblank.c > @@ -1182,10 +1182,12 @@ static int __enable_vblank(struct drm_device *dev= , unsigned int pipe) > static int drm_vblank_enable(struct drm_device *dev, unsigned int pipe) > { > struct drm_vblank_crtc *vblank =3D drm_vblank_crtc(dev, pipe); > - int ret =3D 0; > + int ret; > =20 > assert_spin_locked(&dev->vbl_lock); > =20 > +retry: > + ret =3D 0; > spin_lock(&dev->vblank_time_lock); > =20 > if (!vblank->enabled) { > @@ -1199,7 +1201,11 @@ static int drm_vblank_enable(struct drm_device *de= v, unsigned int pipe) > ret =3D __enable_vblank(dev, pipe); > drm_dbg_core(dev, "enabling vblank on crtc %u, ret: %d\n", > pipe, ret); > - if (ret) { > + if (ret =3D=3D -EDEADLK) { > + spin_unlock(&dev->vblank_time_lock); > + cpu_relax(); > + goto retry; [Severity: High] Can this retry loop still deadlock when outer locks are held across drm_vblank_enable()? When drm_crtc_vblank_start_timer() returns -EDEADLK because drm_vblank_timer_function() is currently executing on another CPU, drm_vblank_enable() only releases dev->vblank_time_lock while continuing to hold dev->vbl_lock and any caller-held locks. However, the running timer callback drm_vblank_timer_function() calls into drm_handle_vblank(), which acquires dev->event_lock before dev->vblank_time_lock, and can also call vblank_disable_fn(), which acquires dev->vbl_lock: drivers/gpu/drm/drm_vblank.c:drm_handle_vblank() { ... spin_lock_irqsave(&dev->event_lock, irqflags); spin_lock(&dev->vblank_time_lock); ... if (disable_irq) vblank_disable_fn(&vblank->disable_timer); } In addition, vkms_crtc_handle_vblank_timeout() acquires vkms_output->lock before calling drm_crtc_handle_vblank(): drivers/gpu/drm/vkms/vkms_crtc.c:vkms_crtc_handle_vblank_timeout() { ... spin_lock(&output->lock); ret =3D drm_crtc_handle_vblank(crtc); ... } Callers that enable vblank during atomic flush or work scheduling hold these outer locks before calling drm_crtc_vblank_get() -> drm_vblank_get() -> drm_vblank_enable(). For example, drm_crtc_vblank_atomic_flush() holds dev->event_lock: drivers/gpu/drm/drm_vblank_helper.c:drm_crtc_vblank_atomic_flush() { ... spin_lock_irq(&dev->event_lock); ... if (event) { if (drm_crtc_vblank_get(crtc) =3D=3D 0) drm_crtc_arm_vblank_event(crtc, event); ... spin_unlock_irq(&dev->event_lock); } Similarly, vkms_crtc_atomic_begin() and vkms_crtc_atomic_flush() hold both vkms_output->lock and dev->event_lock across drm_crtc_vblank_get(), and drm_vblank_get() always holds dev->vbl_lock across drm_vblank_enable(). If drm_vblank_timer_function() already read a non-zero vtimer->interval bef= ore a prior disable cleared it, and is now waiting on vkms_output->lock, dev->event_lock, or dev->vbl_lock, won't CPU A spin forever in the retry lo= op in drm_vblank_enable() while holding the very locks that the timer callback= on CPU B needs to finish? Also, because vtimer->timer is set up with HRTIMER_MODE_REL rather than HRTIMER_MODE_REL_HARD, on CONFIG_PREEMPT_RT the timer callback runs in soft= irq context. If a task calling drm_vblank_enable() preempts the timer callback = on the same CPU and spins with cpu_relax() while holding spinlocks, can the preempted timer callback ever complete? --=20 Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20261102143919.1672= 293-1-tzimmermann@suse.de?part=3D1