Re: [Linaro-mm-sig] [PATCH] dma-buf/poll: Get a file reference for outstanding fence callbacks

[Christian König]

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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Michel Dänzer <mdaenzer@xxxxxxxxxx>
> > > ---
> > >    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@xxxxxxx>

Thanks,
Christian.