linux-kernel - Re: [PATCH] dma-buf: insert memory barrier before updating num

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Message-ID: <20250513022914.GA2330982@au1-maretx-p37.eng.sarc.samsung.com>
Date: Mon, 12 May 2025 21:29:14 -0500
From: Hyejeong Choi <hjeong.choi@...sung.com>
To: Christian König <christian.koenig@....com>,
	sumit.semwal@...aro.org
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	hjeong.choi@...sung.com
Subject: Re: [PATCH] dma-buf: insert memory barrier before updating
 num_fences

Thank you for the review.
I re-sent updated patch, please take a look.

Best regards,
Hyejeong


On Mon, May 12, 2025 at 03:16:33PM +0200, Christian König wrote:
> On 5/12/25 13:12, Hyejeong Choi wrote:
> > smp_store_mb() inserts memory barrier after storing operation.
> > It is different with what the comment is originally aiming so Null
> > pointer dereference can be happened if memory update is reordered.
> > 
> > Signed-off-by: Hyejeong Choi <hjeong.choi@...sung.com>
> > ---
> >  drivers/dma-buf/dma-resv.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> > index 5f8d010516f0..52af5c7430da 100644
> > --- a/drivers/dma-buf/dma-resv.c
> > +++ b/drivers/dma-buf/dma-resv.c
> > @@ -320,8 +320,9 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
> >  	count++;
> >  
> >  	dma_resv_list_set(fobj, i, fence, usage);
> > -	/* pointer update must be visible before we extend the num_fences */
> > -	smp_store_mb(fobj->num_fences, count);
> > +	/* fence update must be visible before we extend the num_fences */
> > +	smp_wmb();
> > +	WRITE_ONCE(fobj->num_fences, count);
> 
> The WRITE_ONCE isn't necessary since smp_wmb() implies a compiler barrier, but apart from that really good catch.
> 
> Can you modify the patch and re-send? I will be pushing it to -fixes ASAP.
> 
> Regards,
> Christian.
> 
> >  }
> >  EXPORT_SYMBOL(dma_resv_add_fence);
> >  
> > 
> > 
> 
>