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Subject: Re: [Linaro-mm-sig] [PATCH] dma-buf/poll: Get a file reference for
 outstanding fence callbacks
To:     =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
Cc:     linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org
References: <20210723075857.4065-1-michel@daenzer.net>
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 <4cf94f59-f953-f5d7-9901-cfe5fd63bfbc@daenzer.net>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
Message-ID: <884050b3-5e7d-c00b-5467-290cfc57e0ea@gmail.com>
Date:   Fri, 23 Jul 2021 10:22:34 +0200
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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,
Christian.