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From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
Subject: Re: [PATCH] dma-buf/poll: Get a file reference for outstanding fence
 callbacks
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Date:   Fri, 23 Jul 2021 10:19:49 +0200
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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.


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