Re: [PATCH 1/7] dma-buf: fix inconsistent debug print

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jun 02, 2021 at 01:17:08PM +0200, Christian König wrote:
> The code tries to acquire the rcu protected fence list, but then ignores
> individual fences which have been modified while holding the rcu.
> 
> Stop that madness and just note cleanly that the list was concurrently modified.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>

Yeah it's debugfs, it's better not to be fancy here and if you race you
can just re-grab it all.

What's worse, we do grab the dma_resv_lock, which means no one should be
able to race with us. I think 100% right thing here is actually to drop
the rcu_read_lock too, and switch over to rcu_dereference_protected().

And also drop the seqcount check, that would be a bug. seqcount is only
to get a consistent snapshot of all fences on the read (i.e. protected by
rcu only) section. We hold the write lock with dma_resv_lock here.

Cheers, Daniel

> ---
>  drivers/dma-buf/dma-buf.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index eadd1eaa2fb5..d3b4e370dbc1 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1383,22 +1383,17 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>  				buf_obj->name ?: "");
>  
>  		robj = buf_obj->resv;
> -		while (true) {
> -			seq = read_seqcount_begin(&robj->seq);
> -			rcu_read_lock();
> -			fobj = rcu_dereference(robj->fence);
> -			shared_count = fobj ? fobj->shared_count : 0;
> -			fence = rcu_dereference(robj->fence_excl);
> -			if (!read_seqcount_retry(&robj->seq, seq))
> -				break;
> -			rcu_read_unlock();
> -		}
> -
> +		seq = read_seqcount_begin(&robj->seq);
> +		rcu_read_lock();
> +		fence = rcu_dereference(robj->fence_excl);
>  		if (fence)
>  			seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",
>  				   fence->ops->get_driver_name(fence),
>  				   fence->ops->get_timeline_name(fence),
>  				   dma_fence_is_signaled(fence) ? "" : "un");
> +
> +		fobj = rcu_dereference(robj->fence);
> +		shared_count = fobj ? fobj->shared_count : 0;
>  		for (i = 0; i < shared_count; i++) {
>  			fence = rcu_dereference(fobj->shared[i]);
>  			if (!dma_fence_get_rcu(fence))
> @@ -1410,6 +1405,8 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>  			dma_fence_put(fence);
>  		}
>  		rcu_read_unlock();
> +		if (read_seqcount_retry(&robj->seq, seq))
> +			seq_printf(s, "\tFences concurrently modified\n");
>  
>  		seq_puts(s, "\tAttached Devices:\n");
>  		attach_count = 0;
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch