From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-14.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8CFBAC11F6A for ; Wed, 30 Jun 2021 10:39:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7204C61A2B for ; Wed, 30 Jun 2021 10:39:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234220AbhF3Klq (ORCPT ); Wed, 30 Jun 2021 06:41:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59680 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229882AbhF3Klm (ORCPT ); Wed, 30 Jun 2021 06:41:42 -0400 Received: from mail-pg1-x52d.google.com (mail-pg1-x52d.google.com [IPv6:2607:f8b0:4864:20::52d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9C159C061756; Wed, 30 Jun 2021 03:39:13 -0700 (PDT) Received: by mail-pg1-x52d.google.com with SMTP id e33so1834999pgm.3; Wed, 30 Jun 2021 03:39:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=V8jwTpsTUaZt0njscvuAxfROwzp0IFoc0SCuX06d7TM=; b=A4uaQ8Jweqt+IDD2UcFYCIjdvm9MAs2YcW//VbQIdieWCf6I8TyUQaDf9vo+BYrh00 hOJT67oN6MlpNhpFjZsDleZ2ponWegv7/QV5anC8YKQJhJg6dk0pMsofSlX4f1QnOEJi ZiA9FD7GC6+R53lqaN0MfkZbF3CTp8+u+MOYBnaHxr20HNf2/IVW/shM4jBfGdg2peIy R32+lFptQxk9aowaSTh5wYi+vs5HPmGVl7X9RZx0CEA5/THAcqfAlSod2Kz1lhV9daRy cYYYRdYIrqIeVEB4I3z/MbZ0AqjStweU8dbWeyM0Ax6bxwjjUc2lPgl2vX95C9dVlZ2c +sFA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=V8jwTpsTUaZt0njscvuAxfROwzp0IFoc0SCuX06d7TM=; b=NfLCBIBjFuBxszkVNFiY7MJhwhR/WrHTePlKyWq0qyZwmfs/S41GKnxCUep08g+Khn yOFwssNO1M51JgnhPDiyPMMAoQJZEesP8Z07XTfkBnOgHb6W8aXJEV9ho6814849fIBI tLgLXSj028yiwmBx9Qj5+I0QMLgNMjXEOIGAf0HMNTzpnEryrIX26IkT0mp77ArjLUMT CKPfK653u1VjKMScnKCE+hPAohr9w/2qc7gVKGKhOfos5T7hYYdykSM6B73eccEt4V2R 8DdHcSM5qKAApAYgFWfAlWtleLkn9zN5uq62JHIYATe0AlGGHhAFAQwDc2/mOB4q8HIK DGBg== X-Gm-Message-State: AOAM5306hU9cqejoVPzWfb6E0j1ASbinVhfOHpEfvDrWIAQ3JztcDZRk 8irnBp+eidI3mCn0W3ttu5o= X-Google-Smtp-Source: ABdhPJxcTo0OsttcS8bBHwiDruzeuUn5E74ttj0gp2t9Rno4jJ1J7WhiNNr5hEe6S5ee53Wp8NkwSw== X-Received: by 2002:a63:381:: with SMTP id 123mr32860371pgd.130.1625049552920; Wed, 30 Jun 2021 03:39:12 -0700 (PDT) Received: from [192.168.1.237] ([118.200.190.93]) by smtp.gmail.com with ESMTPSA id z26sm17600858pfk.112.2021.06.30.03.39.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 30 Jun 2021 03:39:12 -0700 (PDT) Subject: Re: [PATCH v5 3/3] drm: protect drm_master pointers in drm_lease.c To: Daniel Vetter Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Dave Airlie , Sumit Semwal , =?UTF-8?Q?Christian_K=c3=b6nig?= , dri-devel , intel-gfx , Linux Kernel Mailing List , "open list:DMA BUFFER SHARING FRAMEWORK" , "moderated list:DMA BUFFER SHARING FRAMEWORK" , Shuah Khan , Greg KH , linux-kernel-mentees@lists.linuxfoundation.org, Emil Velikov References: <20210629033706.20537-1-desmondcheongzx@gmail.com> <20210629033706.20537-4-desmondcheongzx@gmail.com> From: Desmond Cheong Zhi Xi Message-ID: <524cdcd8-203d-837f-99dc-0c2b3ea3bda3@gmail.com> Date: Wed, 30 Jun 2021 18:39:07 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/6/21 4:02 pm, Daniel Vetter wrote: > On Wed, Jun 30, 2021 at 9:18 AM Desmond Cheong Zhi Xi > wrote: >> >> On 30/6/21 12:07 am, Daniel Vetter wrote: >>> On Tue, Jun 29, 2021 at 11:37:06AM +0800, Desmond Cheong Zhi Xi wrote: >>>> Currently, direct copies of drm_file->master pointers should be >>>> protected by drm_device.master_mutex when being dereferenced. This is >>>> because drm_file->master is not invariant for the lifetime of >>>> drm_file. If drm_file is not the creator of master, then >>>> drm_file->is_master is false, and a call to drm_setmaster_ioctl will >>>> invoke drm_new_set_master, which then allocates a new master for >>>> drm_file and puts the old master. >>>> >>>> Thus, without holding drm_device.master_mutex, the old value of >>>> drm_file->master could be freed while it is being used by another >>>> concurrent process. >>>> >>>> In drm_lease.c, there are multiple instances where drm_file->master is >>>> accessed and dereferenced while drm_device.master_mutex is not >>>> held. This makes drm_lease.c vulnerable to use-after-free bugs. >>>> >>>> We address this issue in 3 ways: >>>> >>>> 1. Clarify in the kerneldoc that drm_file->master is protected by >>>> drm_device.master_mutex. >>>> >>>> 2. Add a new drm_file_get_master() function that calls drm_master_get >>>> on drm_file->master while holding on to drm_device.master_mutex. Since >>>> drm_master_get increments the reference count of master, this >>>> prevents master from being freed until we unreference it with >>>> drm_master_put. >>>> >>>> 3. In each case where drm_file->master is directly accessed and >>>> eventually dereferenced in drm_lease.c, we wrap the access in a call >>>> to the new drm_file_get_master function, then unreference the master >>>> pointer once we are done using it. >>>> >>>> Reported-by: Daniel Vetter >>>> Signed-off-by: Desmond Cheong Zhi Xi >>> >>> Series looks very nice, let's see what intel-gfx-ci says. You should get a >>> mail, but results are also here: >>> >>> https://patchwork.freedesktop.org/series/91969/#rev2 >>> >>> One tiny comment below. >>> >>>> --- >>>> drivers/gpu/drm/drm_auth.c | 25 ++++++++++++ >>>> drivers/gpu/drm/drm_lease.c | 77 +++++++++++++++++++++++++++---------- >>>> include/drm/drm_auth.h | 1 + >>>> include/drm/drm_file.h | 15 ++++++-- >>>> 4 files changed, 95 insertions(+), 23 deletions(-) >>>> >>>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c >>>> index ab1863c5a5a0..c36a0b72be26 100644 >>>> --- a/drivers/gpu/drm/drm_auth.c >>>> +++ b/drivers/gpu/drm/drm_auth.c >>>> @@ -384,6 +384,31 @@ struct drm_master *drm_master_get(struct drm_master *master) >>>> } >>>> EXPORT_SYMBOL(drm_master_get); >>>> >>>> +/** >>>> + * drm_file_get_master - reference &drm_file.master of @file_priv >>>> + * @file_priv: DRM file private >>>> + * >>>> + * Increments the reference count of @file_priv's &drm_file.master and returns >>>> + * the &drm_file.master. If @file_priv has no &drm_file.master, returns NULL. >>>> + * >>>> + * Master pointers returned from this function should be unreferenced using >>>> + * drm_master_put(). >>>> + */ >>>> +struct drm_master *drm_file_get_master(struct drm_file *file_priv) >>>> +{ >>>> + struct drm_master *master = NULL; >>>> + >>>> + mutex_lock(&file_priv->minor->dev->master_mutex); >>>> + if (!file_priv->master) >>>> + goto unlock; >>>> + master = drm_master_get(file_priv->master); >>>> + >>>> +unlock: >>>> + mutex_unlock(&file_priv->minor->dev->master_mutex); >>>> + return master; >>>> +} >>>> +EXPORT_SYMBOL(drm_file_get_master); >>>> + >>>> static void drm_master_destroy(struct kref *kref) >>>> { >>>> struct drm_master *master = container_of(kref, struct drm_master, refcount); >>>> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c >>>> index 00fb433bcef1..cdcc87fa9685 100644 >>>> --- a/drivers/gpu/drm/drm_lease.c >>>> +++ b/drivers/gpu/drm/drm_lease.c >>>> @@ -106,10 +106,19 @@ static bool _drm_has_leased(struct drm_master *master, int id) >>>> */ >>>> bool _drm_lease_held(struct drm_file *file_priv, int id) >>>> { >>>> - if (!file_priv || !file_priv->master) >>>> + bool ret; >>>> + struct drm_master *master; >>>> + >>>> + if (!file_priv) >>>> return true; >>>> >>>> - return _drm_lease_held_master(file_priv->master, id); >>>> + master = drm_file_get_master(file_priv); >>>> + if (master == NULL) >>>> + return true; >>>> + ret = _drm_lease_held_master(master, id); >>>> + drm_master_put(&master); >>>> + >>>> + return ret; >>>> } >>>> >>>> /** >>>> @@ -128,13 +137,20 @@ bool drm_lease_held(struct drm_file *file_priv, int id) >>>> struct drm_master *master; >>>> bool ret; >>>> >>>> - if (!file_priv || !file_priv->master || !file_priv->master->lessor) >>>> + if (!file_priv) >>>> return true; >>>> >>>> - master = file_priv->master; >>>> + master = drm_file_get_master(file_priv); >>>> + if (master == NULL) >>>> + return true; >>>> + if (!master->lessor) { >>>> + drm_master_put(&master); >>>> + return true; >>>> + } >>>> mutex_lock(&master->dev->mode_config.idr_mutex); >>>> ret = _drm_lease_held_master(master, id); >>>> mutex_unlock(&master->dev->mode_config.idr_mutex); >>>> + drm_master_put(&master); >>>> return ret; >>>> } >>>> >>>> @@ -154,10 +170,16 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in) >>>> int count_in, count_out; >>>> uint32_t crtcs_out = 0; >>>> >>>> - if (!file_priv || !file_priv->master || !file_priv->master->lessor) >>>> + if (!file_priv) >>>> return crtcs_in; >>>> >>>> - master = file_priv->master; >>>> + master = drm_file_get_master(file_priv); >>>> + if (master == NULL) >>>> + return crtcs_in; >>>> + if (!master->lessor) { >>>> + drm_master_put(&master); >>>> + return crtcs_in; >>>> + } >>>> dev = master->dev; >>>> >>>> count_in = count_out = 0; >>>> @@ -176,6 +198,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in) >>>> count_in++; >>>> } >>>> mutex_unlock(&master->dev->mode_config.idr_mutex); >>>> + drm_master_put(&master); >>>> return crtcs_out; >>>> } >>>> >>>> @@ -489,7 +512,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> size_t object_count; >>>> int ret = 0; >>>> struct idr leases; >>>> - struct drm_master *lessor = lessor_priv->master; >>>> + struct drm_master *lessor; >>>> struct drm_master *lessee = NULL; >>>> struct file *lessee_file = NULL; >>>> struct file *lessor_file = lessor_priv->filp; >>>> @@ -501,12 +524,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> if (!drm_core_check_feature(dev, DRIVER_MODESET)) >>>> return -EOPNOTSUPP; >>>> >>>> - /* Do not allow sub-leases */ >>>> - if (lessor->lessor) { >>>> - DRM_DEBUG_LEASE("recursive leasing not allowed\n"); >>>> - return -EINVAL; >>>> - } >>>> - >>>> /* need some objects */ >>>> if (cl->object_count == 0) { >>>> DRM_DEBUG_LEASE("no objects in lease\n"); >>>> @@ -518,12 +535,22 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> return -EINVAL; >>>> } >>>> >>>> + lessor = drm_file_get_master(lessor_priv); >>>> + /* Do not allow sub-leases */ >>>> + if (lessor->lessor) { >>>> + DRM_DEBUG_LEASE("recursive leasing not allowed\n"); >>>> + ret = -EINVAL; >>>> + goto out_lessor; >>>> + } >>>> + >>>> object_count = cl->object_count; >>>> >>>> object_ids = memdup_user(u64_to_user_ptr(cl->object_ids), >>>> array_size(object_count, sizeof(__u32))); >>>> - if (IS_ERR(object_ids)) >>>> - return PTR_ERR(object_ids); >>>> + if (IS_ERR(object_ids)) { >>>> + ret = PTR_ERR(object_ids); >>>> + goto out_lessor; >>>> + } >>>> >>>> idr_init(&leases); >>>> >>>> @@ -534,14 +561,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> if (ret) { >>>> DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret); >>>> idr_destroy(&leases); >>>> - return ret; >>>> + goto out_lessor; >>>> } >>>> >>>> /* Allocate a file descriptor for the lease */ >>>> fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK)); >>>> if (fd < 0) { >>>> idr_destroy(&leases); >>>> - return fd; >>>> + ret = fd; >>>> + goto out_lessor; >>>> } >>>> >>>> DRM_DEBUG_LEASE("Creating lease\n"); >>>> @@ -577,6 +605,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> /* Hook up the fd */ >>>> fd_install(fd, lessee_file); >>>> >>>> + drm_master_put(&lessor); >>>> DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); >>>> return 0; >>>> >>>> @@ -586,6 +615,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, >>>> out_leases: >>>> put_unused_fd(fd); >>>> >>>> +out_lessor: >>>> + drm_master_put(&lessor); >>>> DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret); >>>> return ret; >>>> } >>>> @@ -608,7 +639,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev, >>>> struct drm_mode_list_lessees *arg = data; >>>> __u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr); >>>> __u32 count_lessees = arg->count_lessees; >>>> - struct drm_master *lessor = lessor_priv->master, *lessee; >>>> + struct drm_master *lessor, *lessee; >>>> int count; >>>> int ret = 0; >>>> >>>> @@ -619,6 +650,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev, >>>> if (!drm_core_check_feature(dev, DRIVER_MODESET)) >>>> return -EOPNOTSUPP; >>>> >>>> + lessor = drm_file_get_master(lessor_priv); >>>> DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id); >>>> >>>> mutex_lock(&dev->mode_config.idr_mutex); >>>> @@ -642,6 +674,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev, >>>> arg->count_lessees = count; >>>> >>>> mutex_unlock(&dev->mode_config.idr_mutex); >>>> + drm_master_put(&lessor); >>>> >>>> return ret; >>>> } >>>> @@ -661,7 +694,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev, >>>> struct drm_mode_get_lease *arg = data; >>>> __u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr); >>>> __u32 count_objects = arg->count_objects; >>>> - struct drm_master *lessee = lessee_priv->master; >>>> + struct drm_master *lessee; >>>> struct idr *object_idr; >>>> int count; >>>> void *entry; >>>> @@ -675,6 +708,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev, >>>> if (!drm_core_check_feature(dev, DRIVER_MODESET)) >>>> return -EOPNOTSUPP; >>>> >>>> + lessee = drm_file_get_master(lessee_priv); >>>> DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id); >>>> >>>> mutex_lock(&dev->mode_config.idr_mutex); >>>> @@ -702,6 +736,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev, >>>> arg->count_objects = count; >>>> >>>> mutex_unlock(&dev->mode_config.idr_mutex); >>>> + drm_master_put(&lessee); >>>> >>>> return ret; >>>> } >>>> @@ -720,7 +755,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev, >>>> void *data, struct drm_file *lessor_priv) >>>> { >>>> struct drm_mode_revoke_lease *arg = data; >>>> - struct drm_master *lessor = lessor_priv->master; >>>> + struct drm_master *lessor; >>>> struct drm_master *lessee; >>>> int ret = 0; >>>> >>>> @@ -730,6 +765,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev, >>>> if (!drm_core_check_feature(dev, DRIVER_MODESET)) >>>> return -EOPNOTSUPP; >>>> >>>> + lessor = drm_file_get_master(lessor_priv); >>>> mutex_lock(&dev->mode_config.idr_mutex); >>>> >>>> lessee = _drm_find_lessee(lessor, arg->lessee_id); >>>> @@ -750,6 +786,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev, >>>> >>>> fail: >>>> mutex_unlock(&dev->mode_config.idr_mutex); >>>> + drm_master_put(&lessor); >>>> >>>> return ret; >>>> } >>>> diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h >>>> index 6bf8b2b78991..f99d3417f304 100644 >>>> --- a/include/drm/drm_auth.h >>>> +++ b/include/drm/drm_auth.h >>>> @@ -107,6 +107,7 @@ struct drm_master { >>>> }; >>>> >>>> struct drm_master *drm_master_get(struct drm_master *master); >>>> +struct drm_master *drm_file_get_master(struct drm_file *file_priv); >>>> void drm_master_put(struct drm_master **master); >>>> bool drm_is_current_master(struct drm_file *fpriv); >>>> >>>> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h >>>> index b81b3bfb08c8..e9931fca4ab7 100644 >>>> --- a/include/drm/drm_file.h >>>> +++ b/include/drm/drm_file.h >>>> @@ -226,9 +226,18 @@ struct drm_file { >>>> /** >>>> * @master: >>>> * >>>> - * Master this node is currently associated with. Only relevant if >>>> - * drm_is_primary_client() returns true. Note that this only >>>> - * matches &drm_device.master if the master is the currently active one. >>>> + * Master this node is currently associated with. Protected by struct >>>> + * &drm_device.master_mutex. >>>> + * >>>> + * Only relevant if drm_is_primary_client() returns true. Note that >>>> + * this only matches &drm_device.master if the master is the currently >>>> + * active one. >>>> + * >>>> + * When obtaining a copy of this pointer, it is recommended to either >>> >>> I found this a bit confusing, since I generally don't think of >>> dereferencing the pointer as "taking a copy". That's more for the entire >>> datastructure when you have a memcpy() call, or kmemdup() or something >>> like that. Also "it is recommended" is a bit weak if you get a >>> use-after-free if you dont :-) >>> >>> So instead "When dererencing this pointer either hold ... or use >>> drm_file_get_master() ...." >>> >>> Cheers, Daniel >>> >>>> + * hold struct &drm_device.master_mutex for the duration of the >>>> + * pointer's use, or to use drm_file_get_master() if struct >>>> + * &drm_device.master_mutex is not currently held and there is no other >>>> + * need to hold it. This prevents @master from being freed during use. >>>> * >>>> * See also @authentication and @is_master and the :ref:`section on >>>> * primary nodes and authentication `. >>>> -- >>>> 2.25.1 >>>> >>> >> >> Hi Daniel, >> >> Thanks for the suggestion, I'll clarify the kerneldoc accordingly. >> >> Regarding the results from intel-gfx-ci, it seems that the patch is >> inverting the lock hierarchy for >> &dev->master_mutex --> &dev->mode_config.idr_mutex >> >> Currently the dmesg warnings share a common call trace: >> drm_file_get_master+0x1b/0x70 >> _drm_lease_held+0x21/0x70 >> __drm_mode_object_find+0xd1/0xe0 >> >> Looking at the functions that lock &dev->mode_config.idr_mutex, this >> should be the only instance of this lock order inversion. >> >> I'm thinking the call to _drm_lease_held can be moved outside of the >> &dev->mode_config.idr_mutex lock in __drm_mode_object. Any thoughts? > > Uh very annoying. One of the callers of this is the atomic ioctl, > where we're calling this while holding drm_modeset_lock. The nesting > hierarchy is, from outermost lock to innermost: dev->master_mutex -> > dev->mode_config.mutex -> drm_modeset_lock. So I think we'll again > have an inversion, just moved it a bit :-( > > But I'm also not 100% sure, so maybe type it up and see what happens? > Otherwise I think we need to figure out a solution for how we can > check leases without having to take the dev->master_mutex that > serializes a lot more things ... I think the fundamental problem we > have here is that dev->master_mutex serves 2 purposes: a) protecting > the pointers and just data consistency and b) synchronizing against > concurrent master changes where we think that's required. It's the > latter (through the fbdev emulation code) that causes all the > inversion problems, a) it's could easily nest very deeply in other > locks. > Ah I see, that sounds thorny. Just from inspecting the code I can't find the inversion, so I'll write up the v6 series for testing first. >> diff --git a/drivers/gpu/drm/drm_mode_object.c >> b/drivers/gpu/drm/drm_mode_object.c >> index b26588b52795..63d35f1f98dd 100644 >> --- a/drivers/gpu/drm/drm_mode_object.c >> +++ b/drivers/gpu/drm/drm_mode_object.c >> @@ -146,16 +146,18 @@ struct drm_mode_object >> *__drm_mode_object_find(struct drm_device *dev, >> if (obj && obj->id != id) >> obj = NULL; >> >> - if (obj && drm_mode_object_lease_required(obj->type) && >> - !_drm_lease_held(file_priv, obj->id)) >> - obj = NULL; >> - >> if (obj && obj->free_cb) { >> if (!kref_get_unless_zero(&obj->refcount)) >> obj = NULL; >> } >> mutex_unlock(&dev->mode_config.idr_mutex); >> >> + if (obj && drm_mode_object_lease_required(obj->type) && >> + !_drm_lease_held(file_priv, obj->id)) { >> + drm_mode_object_put(obj); >> + obj = NULL; >> + } > > Irrespective of all this I think this change here makes sense since in > untangels the master stuff from the lookup idr, and that's always > good. Maybe do this hunk as a separate patch, and I'll apply that as > prep work? > -Daniel > Sounds good to me, I'll add the new patch to the series.