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From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
Subject: Re: [Linaro-mm-sig] [PATCH] dma-buf/poll: Get a file reference for
 outstanding fence callbacks
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On 2021-07-23 10:22, Christian König wrote:
> Am 23.07.21 um 10:19 schrieb Michel Dänzer:
>> On 2021-07-23 10:04 a.m., Christian König wrote:
>>> Am 23.07.21 um 09:58 schrieb Michel Dänzer:
>>>> From: Michel Dänzer <mdaenzer@redhat.com>
>>>>
>>>> This makes sure we don't hit the
>>>>
>>>>      BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
>>>>
>>>> in dma_buf_release, which could be triggered by user space closing the
>>>> dma-buf file description while there are outstanding fence callbacks
>>>> from dma_buf_poll.
>>> I was also wondering the same thing while working on this, but then thought that the poll interface would take care of this.
>> I was able to hit the BUG_ON with https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1880 .
>>
>>
>>>> Cc: stable@vger.kernel.org
>>>> Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
>>>> ---
>>>>    drivers/dma-buf/dma-buf.c | 18 ++++++++++++------
>>>>    1 file changed, 12 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>>> index 6c520c9bd93c..ec25498a971f 100644
>>>> --- a/drivers/dma-buf/dma-buf.c
>>>> +++ b/drivers/dma-buf/dma-buf.c
>>>> @@ -65,12 +65,9 @@ static void dma_buf_release(struct dentry *dentry)
>>>>        BUG_ON(dmabuf->vmapping_counter);
>>>>          /*
>>>> -     * Any fences that a dma-buf poll can wait on should be signaled
>>>> -     * before releasing dma-buf. This is the responsibility of each
>>>> -     * driver that uses the reservation objects.
>>>> -     *
>>>> -     * If you hit this BUG() it means someone dropped their ref to the
>>>> -     * dma-buf while still having pending operation to the buffer.
>>>> +     * If you hit this BUG() it could mean:
>>>> +     * * There's a file reference imbalance in dma_buf_poll / dma_buf_poll_cb or somewhere else
>>>> +     * * dmabuf->cb_in/out.active are non-0 despite no pending fence callback
>>>>         */
>>>>        BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
>>>>    @@ -196,6 +193,7 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
>>>>    static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
>>>>    {
>>>>        struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
>>>> +    struct dma_buf *dmabuf = container_of(dcb->poll, struct dma_buf, poll);
>>>>        unsigned long flags;
>>>>          spin_lock_irqsave(&dcb->poll->lock, flags);
>>>> @@ -203,6 +201,8 @@ static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
>>>>        dcb->active = 0;
>>>>        spin_unlock_irqrestore(&dcb->poll->lock, flags);
>>>>        dma_fence_put(fence);
>>>> +    /* Paired with get_file in dma_buf_poll */
>>>> +    fput(dmabuf->file);
>>> Is calling fput() in interrupt context ok? IIRC that could potentially sleep.
>> Looks fine AFAICT: It has
>>
>>         if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
>>
>> and as a fallback for that, it adds the file to a lock-less delayed_fput_list which is processed by a workqueue.
> 
> Ah, yes that makes sense.
> 
> Fell free to add Reviewed-by: Christian König <christian.koenig@amd.com>

Thanks! AFAICT this fix can be merged now for 5.16?


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer