[PATCH v5] dma-buf/sync-file: Defer creation of sync_file->name

[Gustavo Padovan]

2017-05-22 Daniel Vetter <daniel at ffwll.ch>:

> On Tue, May 16, 2017 at 12:10:42PM +0100, Chris Wilson wrote:
> > Constructing the name takes the majority of the time for allocating a
> > sync_file to wrap a fence, and the name is very rarely used (only via
> > the sync_file status user interface). To reduce the impact on the common
> > path (that of creating sync_file to pass around), defer the construction
> > of the name until it is first used.
> > 
> > v2: Update kerneldoc (kbuild test robot)
> > v3: sync_debug.c was peeking at the name
> > v4: Comment upon the potential race between two users of
> > sync_file_get_name() and claim that such a race is below the level of
> > notice. However, to prevent any future nuisance, use a global spinlock
> > to serialize the assignment of the name.
> > v5: Completely avoid the read/write race by only storing the name passed
> > in from the user inside sync_file->user_name and passing in a buffer to
> > dynamically construct the name otherwise.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Sumit Semwal <sumit.semwal at linaro.org>
> > Cc: Gustavo Padovan <gustavo at padovan.org>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: David Herrmann <dh.herrmann at gmail.com>
> > ---
> >  drivers/dma-buf/sync_debug.c |  4 +++-
> >  drivers/dma-buf/sync_file.c  | 39 ++++++++++++++++++++++++++++++++-------
> >  include/linux/sync_file.h    |  5 +++--
> >  3 files changed, 38 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/sync_debug.c b/drivers/dma-buf/sync_debug.c
> > index 4b1731ee7458..59a3b2f8ee91 100644
> > --- a/drivers/dma-buf/sync_debug.c
> > +++ b/drivers/dma-buf/sync_debug.c
> > @@ -132,9 +132,11 @@ static void sync_print_obj(struct seq_file *s, struct sync_timeline *obj)
> >  static void sync_print_sync_file(struct seq_file *s,
> >  				  struct sync_file *sync_file)
> >  {
> > +	char buf[128];
> >  	int i;
> >  
> > -	seq_printf(s, "[%p] %s: %s\n", sync_file, sync_file->name,
> > +	seq_printf(s, "[%p] %s: %s\n", sync_file,
> > +		   sync_file_get_name(sync_file, buf, sizeof(buf)),
> >  		   sync_status_str(dma_fence_get_status(sync_file->fence)));
> >  
> >  	if (dma_fence_is_array(sync_file->fence)) {
> > diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> > index c9eb4997cfcc..d7e219d2669d 100644
> > --- a/drivers/dma-buf/sync_file.c
> > +++ b/drivers/dma-buf/sync_file.c
> > @@ -80,11 +80,6 @@ struct sync_file *sync_file_create(struct dma_fence *fence)
> >  
> >  	sync_file->fence = dma_fence_get(fence);
> >  
> > -	snprintf(sync_file->name, sizeof(sync_file->name), "%s-%s%llu-%d",
> > -		 fence->ops->get_driver_name(fence),
> > -		 fence->ops->get_timeline_name(fence), fence->context,
> > -		 fence->seqno);
> > -
> >  	return sync_file;
> >  }
> >  EXPORT_SYMBOL(sync_file_create);
> > @@ -129,6 +124,36 @@ struct dma_fence *sync_file_get_fence(int fd)
> >  }
> >  EXPORT_SYMBOL(sync_file_get_fence);
> >  
> > +/**
> > + * sync_file_get_name - get the name of the sync_file
> > + * @sync_file:		sync_file to get the fence from
> > + * @buf:		destination buffer to copy sync_file name into
> > + * @len:		available size of destination buffer.
> > + *
> > + * Each sync_file may have a name assigned either by the user (when merging
> > + * sync_files together) or created from the fence it contains. In the latter
> > + * case construction of the name is deferred until use, and so requires
> > + * sync_file_get_name().
> > + *
> > + * Returns: a string representing the name.
> > + */
> > +char *sync_file_get_name(struct sync_file *sync_file, char *buf, int len)
> > +{
> > +	if (sync_file->user_name[0]) {
> > +		strlcpy(buf, sync_file->user_name, len);
> > +	} else {
> > +		struct dma_fence *fence = sync_file->fence;
> > +
> > +		snprintf(buf, len, "%s-%s%llu-%d",
> > +			 fence->ops->get_driver_name(fence),
> > +			 fence->ops->get_timeline_name(fence),
> > +			 fence->context,
> > +			 fence->seqno);
> > +	}
> > +
> > +	return buf;
> > +}
> > +
> >  static int sync_file_set_fence(struct sync_file *sync_file,
> >  			       struct dma_fence **fences, int num_fences)
> >  {
> > @@ -266,7 +291,7 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
> >  		goto err;
> >  	}
> >  
> > -	strlcpy(sync_file->name, name, sizeof(sync_file->name));
> > +	strlcpy(sync_file->user_name, name, sizeof(sync_file->user_name));
> >  	return sync_file;
> >  
> >  err:
> > @@ -419,7 +444,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
> >  	}
> >  
> >  no_fences:
> > -	strlcpy(info.name, sync_file->name, sizeof(info.name));
> > +	sync_file_get_name(sync_file, info.name, sizeof(info.name));
> >  	info.status = dma_fence_is_signaled(sync_file->fence);
> >  	info.num_fences = num_fences;
> >  
> > diff --git a/include/linux/sync_file.h b/include/linux/sync_file.h
> > index d37beefdfbd5..5226d62ae91b 100644
> > --- a/include/linux/sync_file.h
> > +++ b/include/linux/sync_file.h
> > @@ -23,7 +23,7 @@
> >  /**
> >   * struct sync_file - sync file to export to the userspace
> >   * @file:		file representing this fence
> > - * @name:		name of sync_file.  Useful for debugging
> > + * @user_name:		name of sync_file.  Useful for debugging
> >   * @sync_file_list:	membership in global file list
> >   * @wq:			wait queue for fence signaling
> >   * @fence:		fence with the fences in the sync_file
> > @@ -31,7 +31,7 @@
> >   */
> >  struct sync_file {
> >  	struct file		*file;
> > -	char			name[32];
> 
> Maybe inline kerneldoc an explain a bit better what's going on:
> 
> 	/**
> 	 * @user_name:
> 	 *
> 	 * Name of the sync file provided by userspace, for merged fences.
> 	 * Otherwise generated through driver callbacks (in which case the
> 	 * entire array is 0).
> 	 */
> 
> Might be overkill, so not going to insist at all. Either way:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

I fixed it. Applied with the modification proposed by Daniel. Thanks for
the patch.

Gustavo