linux-kernel - Re: [PATCH 01/19] drm/msm: remove dangling submitqueue references

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Message-ID: <20200901023517.GA54956@uller>
Date:   Tue, 1 Sep 2020 02:35:17 +0000
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linux-arm-msm@...r.kernel.org,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Will Deacon <will@...nel.org>, freedreno@...ts.freedesktop.org,
        Sibi Sankar <sibis@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/19] drm/msm: remove dangling submitqueue references

On Fri 14 Aug 02:40 UTC 2020, Rob Clark wrote:

> From: Rob Clark <robdclark@...omium.org>
> 
> Currently it doesn't matter, since we free the ctx immediately.  But
> when we start refcnt'ing the ctx, we don't want old dangling list
> entries to hang around.
> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> ---
>  drivers/gpu/drm/msm/msm_submitqueue.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_submitqueue.c b/drivers/gpu/drm/msm/msm_submitqueue.c
> index a1d94be7883a..90c9d84e6155 100644
> --- a/drivers/gpu/drm/msm/msm_submitqueue.c
> +++ b/drivers/gpu/drm/msm/msm_submitqueue.c
> @@ -49,8 +49,10 @@ void msm_submitqueue_close(struct msm_file_private *ctx)
>  	 * No lock needed in close and there won't
>  	 * be any more user ioctls coming our way
>  	 */
> -	list_for_each_entry_safe(entry, tmp, &ctx->submitqueues, node)
> +	list_for_each_entry_safe(entry, tmp, &ctx->submitqueues, node) {
> +		list_del(&entry->node);

If you refcount ctx, what does that do for the entries in the submit
queue?

"entry" here is kref'ed, but you're popping it off the list regardless
of the put ends up freeing the object or not - which afaict would mean
leaking the object.


On the other hand, with the current implementation an object with higher
refcount with adjacent objects of single refcount would end up with
dangling pointers after the put. So in itself this change seems like a
net gain, but I'm wondering about the plan described in the commit
message.

Regards,
Bjorn

>  		msm_submitqueue_put(entry);
> +	}
>  }
>  
>  int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
> -- 
> 2.26.2
>