[Intel-gfx] [PATCH] drm: Release filp before global lock

[Thomas Hellström (VMware)]

On 1/22/20 11:00 PM, Chris Wilson wrote:
> Quoting Thomas Hellström (VMware) (2020-01-22 21:52:23)
>> Hi, Chris,
>>
>> On 1/22/20 4:56 PM, Chris Wilson wrote:
>>> The file is not part of the global drm resource and can be released
>>> prior to take the global mutex to drop the open_count (and potentially
>>> close) the drm device.
>>>
>>> However, inside drm_close_helper() there are a number of dev->driver
>>> callbacks that take the drm_device as the first parameter... Worryingly
>>> some of those callbacks may be (implicitly) depending on the global
>>> mutex.
>> I read this as you suspect that there are driver callbacks inside
>> drm_close_helper() that might need the global mutex held? But then it
>> wouldn't be safe to move the lock? Is there a strong motivation for
>> moving the locking in the first place? Also a minor nit below:
> The number of processes stuck on 'D' due to mutex_lock(&global) caught my
> attention while they were cleaning up files. I think everyone else will
> be less impressed if their driver was stuck because i915 was freeing a
> user's filp.

Understood. Perhaps a short motivation in the log message?

>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> ---
>>>    drivers/gpu/drm/drm_file.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
>>> index 92d16724f949..84ed313ee2e9 100644
>>> --- a/drivers/gpu/drm/drm_file.c
>>> +++ b/drivers/gpu/drm/drm_file.c
>>> @@ -438,12 +438,12 @@ int drm_release(struct inode *inode, struct file *filp)
>>>        struct drm_minor *minor = file_priv->minor;
>>>        struct drm_device *dev = minor->dev;
>>>    
>>> -     mutex_lock(&drm_global_mutex);
>>> -
>>>        DRM_DEBUG("open_count = %d\n", dev->open_count);
>> The read of dev->open_count should still be inside the lock to be
>> consistent with the value that is decremented below. Perhaps move the
>> DRM_DEBUG()?
> Sure. Is it even worth a debug?

Probably an old relic. I'm fine with letting it go.

Thanks,

Thomas



> -Chris