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

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Message-ID: <20240827173137.0000404f@Huawei.com>
Date: Tue, 27 Aug 2024 17:31:37 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Ira Weiny <ira.weiny@...el.com>
CC: Davidlohr Bueso <dave@...olabs.net>, 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 12:49:39 -0500
Ira Weiny <ira.weiny@...el.com> 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.
> 
> Furthermore, remove cxl_region_nid() because resource can't be NULL
> while the region is stable.
> 
> Link: https://lore.kernel.org/all/66b4cf539a79b_a36e829416@iweiny-mobl.notmuch/ [0]
> Cc: Huang, Ying <ying.huang@...el.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
Seems a sensible cleanup irrespective of the bug / future issue.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>