linux-kernel - Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan

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Date:   Thu, 25 Jul 2019 14:56:36 +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" <rjw@...ysocki.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in
 acpi_scan_init()

On Wed 24-07-19 16:30:17, David Hildenbrand wrote:
> We end up calling __add_memory() without the device hotplug lock held.
> (I used a local patch to assert in __add_memory() that the
>  device_hotplug_lock is held - I might upstream that as well soon)
> 
> [   26.771684]        create_memory_block_devices+0xa4/0x140
> [   26.772952]        add_memory_resource+0xde/0x200
> [   26.773987]        __add_memory+0x6e/0xa0
> [   26.775161]        acpi_memory_device_add+0x149/0x2b0
> [   26.776263]        acpi_bus_attach+0xf1/0x1f0
> [   26.777247]        acpi_bus_attach+0x66/0x1f0
> [   26.778268]        acpi_bus_attach+0x66/0x1f0
> [   26.779073]        acpi_bus_attach+0x66/0x1f0
> [   26.780143]        acpi_bus_scan+0x3e/0x90
> [   26.780844]        acpi_scan_init+0x109/0x257
> [   26.781638]        acpi_init+0x2ab/0x30d
> [   26.782248]        do_one_initcall+0x58/0x2cf
> [   26.783181]        kernel_init_freeable+0x1bd/0x247
> [   26.784345]        kernel_init+0x5/0xf1
> [   26.785314]        ret_from_fork+0x3a/0x50
> 
> So perform the locking just like in acpi_device_hotplug().

While playing with the device_hotplug_lock, can we actually document
what it is protecting please? I have a bad feeling that we are adding
this lock just because some other code path does rather than with a good
idea why it is needed. This patch just confirms that. What exactly does
the lock protect from here in an early boot stage.

> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Len Brown <lenb@...nel.org
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  drivers/acpi/scan.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index 0e28270b0fd8..cbc9d64b48dd 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -2204,7 +2204,9 @@ int __init acpi_scan_init(void)
>  	acpi_gpe_apply_masked_gpes();
>  	acpi_update_all_gpes();
>  
> +	lock_device_hotplug();
>  	mutex_lock(&acpi_scan_lock);
> +
>  	/*
>  	 * Enumerate devices in the ACPI namespace.
>  	 */
> @@ -2232,6 +2234,7 @@ int __init acpi_scan_init(void)
>  
>   out:
>  	mutex_unlock(&acpi_scan_lock);
> +	unlock_device_hotplug();
>  	return result;
>  }
>  
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs