Re: Use of copy_from_user in msm_gem_submit.c while holding a spin_lock

From: Rob Clark <robdclark@gmail.com>
To: Vaishali Thakkar <vaishali.thakkar@oracle.com>
Cc: David Airlie <airlied@linux.ie>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	freedreno@lists.freedesktop.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Julia Lawall <julia.lawall@lip6.fr>
Subject: Re: Use of copy_from_user in msm_gem_submit.c while holding a spin_lock
Date: Wed, 17 Aug 2016 11:08:46 -0400	[thread overview]
Message-ID: <CAF6AEGs31L4R4XG_GxDU5u0kC9yCKYpq3k9998b6KH-OPq6CQA@mail.gmail.com> (raw)
In-Reply-To: <57B44D2E.2030301@oracle.com>

On Wed, Aug 17, 2016 at 7:40 AM, Vaishali Thakkar
<vaishali.thakkar@oracle.com> wrote:
> Hello,
>
> I was wondering about the call to copy_from_user in function submit_lookup_objects for drive
> /gpu/drm/msm/msm_gem_submit.c  It calls copy_from_user[1] in a spin_lock, which is not normally
> allowed, due to the possibility of a deadlock.
>
> Is there some reason that I am overlooking why it is OK in this case? Is there some code in the
> same file which ensures that page fault will not occur when we are calling the function holding
> spin_lock?

hmm, probably just that it isn't typical to use a swap file on these
devices (and that lockdep/etc doesn't warn about it)..  I guess we
probably need some sort of slow-path where we drop the lock and try
again in case there would be a fault..

BR,
-R

> Thank you.
>
> [1] http://lxr.free-electrons.com/source/drivers/gpu/drm/msm/msm_gem_submit.c#L85
>
> --
> Vaishali