Re: [PATCH] dma-buf/sync_file: only enable fence signalling during wait

From: Gustavo Padovan
Date: Mon Jul 11 2016 - 16:27:23 EST


2016-07-10 Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>:

> Op 08-07-16 om 17:44 schreef Gustavo Padovan:
> > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> >
> > Signalling doesn't need to be enabled at sync_file creation, it is only
> > required if userspace waiting the fence to signal through poll().
> >
> > Thus we delay fence_add_callback() until poll is called. It only adds the
> > callback the first time poll() is called. This avoid re-adding the same
> > callback multiple times.
> >
> > v2: rebase and update to work with new fence support for sync_file
> >
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> > ---
> > This patch applies on top of my latest sync_file changes to support
> > fence_array: https://lkml.org/lkml/2016/7/4/534
> >
> > drivers/dma-buf/sync_file.c | 23 ++++++++++++++---------
> > include/linux/sync_file.h | 2 ++
> > 2 files changed, 16 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> > index 61a687c..1db4a64 100644
> > --- a/drivers/dma-buf/sync_file.c
> > +++ b/drivers/dma-buf/sync_file.c
> > @@ -86,8 +86,6 @@ struct sync_file *sync_file_create(struct fence *fence)
> > fence->ops->get_timeline_name(fence), fence->context,
> > fence->seqno);
> >
> > - fence_add_callback(fence, &sync_file->cb, fence_check_cb_func);
> > -
> > return sync_file;
> > }
> > EXPORT_SYMBOL(sync_file_create);
> > @@ -269,9 +267,6 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
> > goto err;
> > }
> >
> > - fence_add_callback(sync_file->fence, &sync_file->cb,
> > - fence_check_cb_func);
> > -
> > strlcpy(sync_file->name, name, sizeof(sync_file->name));
> > return sync_file;
> >
> > @@ -286,7 +281,6 @@ static void sync_file_free(struct kref *kref)
> > struct sync_file *sync_file = container_of(kref, struct sync_file,
> > kref);
> >
> > - fence_remove_callback(sync_file->fence, &sync_file->cb);
> > fence_put(sync_file->fence);
> > kfree(sync_file);
> > }
> > @@ -306,13 +300,24 @@ static unsigned int sync_file_poll(struct file *file, poll_table *wait)
> >
> > poll_wait(file, &sync_file->wq, wait);
> >
> > + if (!sync_file->enabled) {
> > + fence_add_callback(sync_file->fence, &sync_file->cb,
> > + fence_check_cb_func);
> > + sync_file->enabled = true;
> > + }
> Won't this blow up completely with 2 threads polling at the same time?

Indeed, using atomic operations on enabled should fix this.

Gustavo