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

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Message-ID: <20160711202713.GC2565@joana>
Date:	Mon, 11 Jul 2016 17:27:13 -0300
From:	Gustavo Padovan <gustavo@...ovan.org>
To:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Daniel Stone <daniels@...labora.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>,
	laurent.pinchart@...asonboard.com, seanpaul@...gle.com,
	marcheu@...gle.com, m.chehab@...sung.com,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
Subject: Re: [PATCH] dma-buf/sync_file: only enable fence signalling during
 wait

2016-07-10 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>:

> Op 08-07-16 om 17:44 schreef Gustavo Padovan:
> > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> >
> > 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@...labora.co.uk>
> > ---
> > 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