linux-kernel - Re: [PATCH v1] drivers/acpi/scan.c: Document why we don't need the device_hotplug

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Date:   Wed, 31 Jul 2019 16:17:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-acpi@...r.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1] drivers/acpi/scan.c: Document why we don't need the
 device_hotplug_lock

On Wed 31-07-19 15:53:06, David Hildenbrand wrote:
> Let's document why the lock is not needed in acpi_scan_init(), right now
> this is not really obvious.
> 
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
> 
> @Andrew, can you drop "drivers/acpi/scan.c: acquire device_hotplug_lock in
> acpi_scan_init()" and add this patch instead? Thanks
> 
> ---
>  drivers/acpi/scan.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index 0e28270b0fd8..8444af6cd514 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -2204,6 +2204,12 @@ int __init acpi_scan_init(void)
>  	acpi_gpe_apply_masked_gpes();
>  	acpi_update_all_gpes();
>  
> +	/*
> +	 * Although we call__add_memory() that is documented to require the
> +	 * device_hotplug_lock, it is not necessary here because this is an
> +	 * early code when userspace or any other code path cannot trigger
> +	 * hotplug/hotunplug operations.
> +	 */
>  	mutex_lock(&acpi_scan_lock);
>  	/*
>  	 * Enumerate devices in the ACPI namespace.
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs