linux-kernel - Re: [PATCH v2] cxl/region: Remove lock from memory notifier callback

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Message-ID: <20240815161749.hb2zzwam5rieswws@offworld>
Date: Thu, 15 Aug 2024 09:17:49 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: Ira Weiny <ira.weiny@...el.com>
Cc: Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dan Williams <dan.j.williams@...el.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cxl/region: Remove lock from memory notifier callback

On Wed, 14 Aug 2024, Ira Weiny wrote:

>In testing Dynamic Capacity Device (DCD) support, a lockdep splat
>revealed an ABBA issue between the memory notifiers and the DCD extent
>processing code.[0]  Changing the lock ordering within DCD proved
>difficult because regions must be stable while searching for the proper
>region and then the device lock must be held to properly notify the DAX
>region driver of memory changes.
>
>Dan points out in the thread that notifiers should be able to trust that
>it is safe to access static data.  Region data is static once the device
>is realized and until it's destruction.  Thus it is better to manage the
>notifiers within the region driver.
>
>Remove the need for a lock by ensuring the notifiers are active only
>during the region's lifetime.

Agreed, this is better.

>Furthermore, remove cxl_region_nid() because resource can't be NULL
>while the region is stable.

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>