linux-kernel - Re: [PATCH net-next] net: ethernet: mtk_wed: annotate RCU release in attach()

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Message-ID: <20250425102050.GO3042781@horms.kernel.org>
Date: Fri, 25 Apr 2025 11:20:50 +0100
From: Simon Horman <horms@...nel.org>
To: Johannes Berg <johannes@...solutions.net>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org,
	Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH net-next] net: ethernet: mtk_wed: annotate RCU release in
 attach()

On Wed, Apr 23, 2025 at 05:08:08PM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> There are some sparse warnings in wifi, and it seems that
> it's actually possible to annotate a function pointer with
> __releases(), making the sparse warnings go away. In a way
> that also serves as documentation that rcu_read_unlock()
> must be called in the attach method, so add that annotation.
> 
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Reviewed-by: Simon Horman <horms@...nel.org>

Thinking out loud:

* Without this patch I see the following, but with this patch I do not.

  .../mt7915/mmio.c:636:5: warning: context imbalance in 'mt7915_mmio_wed_init' - wrong count at exit
  .../mt7996/mmio.c:302:5: warning: context imbalance in 'mt7996_mmio_wed_init' - wrong count at exit

* The only implementation of this callback I found is mtk_wed_attach
  which is already annotated as __releases(RCU);

* The only caller of this callback I could find is mtk_wed_device_attach()
  which takes rcu_read_unlock(). And the the callback needs to release it
  to avoid imbalance.