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

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Message-ID: <66bbc21f7cfd3_216829434@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Tue, 13 Aug 2024 13:29:19 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Ira Weiny <ira.weiny@...el.com>, Davidlohr Bueso <dave@...olabs.net>,
	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>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Ira Weiny
	<ira.weiny@...el.com>
Subject: Re: [PATCH] cxl/region: Remove lock from memory notifier callback

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.
> 
> 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>
> ---
>  drivers/cxl/core/region.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 21ad5f242875..971a314b6b0e 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
[..]
> @@ -2396,7 +2394,6 @@ static int cxl_region_nid(struct cxl_region *cxlr)
>  	struct cxl_region_params *p = &cxlr->params;
>  	struct resource *res;
>  
> -	guard(rwsem_read)(&cxl_region_rwsem);
>  	res = p->res;
>  	if (!res)
>  		return NUMA_NO_NODE;

The cxl_region_nid() helper is now completely unnecessary because not
only is a lock not needed to read cxl_region_params, but p->res is
guaranteed to be non-NULL.

cxl_region_nid() also needs to be killed so that nothing else tries to
use it that might *need* the lock.