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Message-ID: <c9e08f10-035f-63dd-698c-faa93c935cd6@gmail.com>
Date: Tue, 31 Aug 2021 05:04:04 +0800
From: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To: maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
tzimmermann@...e.de, airlied@...ux.ie, sumit.semwal@...aro.org,
christian.koenig@....com, jani.nikula@...ux.intel.com,
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chris@...is-wilson.co.uk, ville.syrjala@...ux.intel.com,
matthew.auld@...el.com, dan.carpenter@...cle.com,
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Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v8 3/7] drm: lock drm_global_mutex earlier in the ioctl
handler
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 <desmondcheongzx@...il.com>
>> ---
>> 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 <daniel.vetter@...ll.ch>
>
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
>>
>
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