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Mon, 30 Aug 2021 14:04:13 -0700 (PDT) Subject: Re: [PATCH v8 3/7] drm: lock drm_global_mutex earlier in the ioctl handler To: maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org, christian.koenig@amd.com, jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com, chris@chris-wilson.co.uk, ville.syrjala@linux.intel.com, matthew.auld@intel.com, dan.carpenter@oracle.com, tvrtko.ursulin@intel.com, matthew.d.roper@intel.com, lucas.demarchi@intel.com, karthik.b.s@intel.com, jose.souza@intel.com, manasi.d.navare@intel.com, airlied@redhat.com, aditya.swarup@intel.com, andrescj@chromium.org, linux-graphics-maintainer@vmware.com, zackr@vmware.com, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org, Daniel Vetter References: <20210826020122.1488002-1-desmondcheongzx@gmail.com> <20210826020122.1488002-4-desmondcheongzx@gmail.com> From: Desmond Cheong Zhi Xi Message-ID: Date: Tue, 31 Aug 2021 05:04:04 +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=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26/8/21 5:58 pm, Daniel Vetter wrote: > On Thu, Aug 26, 2021 at 10:01:18AM +0800, Desmond Cheong Zhi Xi wrote: >> In a future patch, a read lock on drm_device.master_rwsem is >> held in the ioctl handler before the check for ioctl >> permissions. However, this inverts the lock hierarchy of >> drm_global_mutex --> master_rwsem. >> >> To avoid this, we do some prep work to grab the drm_global_mutex >> before checking for ioctl permissions. >> >> Signed-off-by: Desmond Cheong Zhi Xi >> --- >> drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- >> 1 file changed, 9 insertions(+), 9 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c >> index d25713b09b80..158629d88319 100644 >> --- a/drivers/gpu/drm/drm_ioctl.c >> +++ b/drivers/gpu/drm/drm_ioctl.c >> @@ -772,19 +772,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, >> if (drm_dev_is_unplugged(dev)) >> return -ENODEV; >> >> + /* Enforce sane locking for modern driver ioctls. */ >> + if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED)) > > Maybe have a local bool locked_ioctl for this so it's extremely clear it's > the same condition in both? > > Either way: Reviewed-by: Daniel Vetter > Thanks for the suggestion and review. Sounds good, I'll update and send out a new version. (Sorry for delays, been busy with moving) >> + mutex_lock(&drm_global_mutex); >> + >> retcode = drm_ioctl_permit(flags, file_priv); >> if (unlikely(retcode)) >> - return retcode; >> + goto out; >> >> - /* Enforce sane locking for modern driver ioctls. */ >> - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || >> - (flags & DRM_UNLOCKED)) >> - retcode = func(dev, kdata, file_priv); >> - else { >> - mutex_lock(&drm_global_mutex); >> - retcode = func(dev, kdata, file_priv); >> + retcode = func(dev, kdata, file_priv); >> + >> +out: >> + if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED)) >> mutex_unlock(&drm_global_mutex); >> - } >> return retcode; >> } >> EXPORT_SYMBOL(drm_ioctl_kernel); >> -- >> 2.25.1 >> >