linux-kernel - Re: [PATCH v7 05/28] dma-heap: Add proper kref handling on dma-buf heaps

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Message-ID: <CANDhNCqzme0q7vey1jc84T0HJPiOSQF2fkyBfBLntdiyafEzYA@mail.gmail.com>
Date: Mon, 22 Jul 2024 11:06:12 -0700
From: John Stultz <jstultz@...gle.com>
To: Markus Elfring <Markus.Elfring@....de>
Cc: "T. J. Mercier" <tjmercier@...gle.com>, Yong Wu <yong.wu@...iatek.com>, 
	Yunfei Dong <yunfei.dong@...iatek.com>, 
	Project_Global_Chrome_Upstream_Group@...iatek.com, 
	linux-mediatek@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	Angelo Gioacchino Del Regno <angelogioacchino.delregno@...labora.com>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Hans Verkuil <hverkuil-cisco@...all.nl>, 
	Jeffrey Kardatzke <jkardatzke@...gle.com>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Nathan Hebert <nhebert@...omium.org>, 
	Nicolas Dufresne <nicolas.dufresne@...labora.com>, 
	Nícolas F. R. A. Prado <nfraprado@...labora.com>, 
	Sebastian Fricke <sebastian.fricke@...labora.com>, Tomasz Figa <tfiga@...omium.org>, 
	LKML <linux-kernel@...r.kernel.org>, Brian Starkey <Brian.Starkey@....com>, 
	Chen-Yu Tsai <wenst@...omium.org>, Christian König <christian.koenig@....com>, 
	Daniel Vetter <daniel@...ll.ch>, Fritz König <frkoenig@...omium.org>, 
	Hsin-yi Wang <hsinyi@...omium.org>, Matthias Brugger <matthias.bgg@...il.com>, 
	Steve Cho <stevecho@...omium.org>, Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH v7 05/28] dma-heap: Add proper kref handling on dma-buf heaps

On Sat, Jul 20, 2024 at 8:13 AM Markus Elfring <Markus.Elfring@....de> wrote:
>
> …
> > +++ b/drivers/dma-buf/dma-heap.c
> …
> > +static void dma_heap_release(struct kref *ref)
> > +{
> …
> > +     mutex_lock(&heap_list_lock);
> > +     list_del(&heap->list);
> > +     mutex_unlock(&heap_list_lock);
> …
>
> Under which circumstances would you become interested to apply a statement
> like “guard(mutex)(&heap_list_lock);”?
> https://elixir.bootlin.com/linux/v6.10/source/include/linux/mutex.h#L196

This strikes me as a strange place to apply it, as it seems like it
would grow the lock hold time to the entire scope of the function
unless one created a subscope for just the list_del, at which point
you're not saving much or really improving readability.  I definitely
think guard usage is very interesting in places where locks are
released in multiple exit paths, etc. but this is a very trivial and
straightforward lock/unlock usage, so I fret I don't quite understand
the suggestion.

thanks
-john