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[195.135.220.15]) by mx.google.com with ESMTPS id a5si18694547ejb.103.2019.07.31.05.53.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 31 Jul 2019 05:53:36 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 512F7AFEC; Wed, 31 Jul 2019 12:53:35 +0000 (UTC) Date: Wed, 31 Jul 2019 14:53:34 +0200 From: Michal Hocko To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-acpi@vger.kernel.org, "Rafael J . Wysocki" , Oscar Salvador , Andrew Morton Subject: Re: [PATCH v1] drivers/acpi/scan.c: Fixup "acquire device_hotplug_lock in acpi_scan_init()" Message-ID: <20190731125334.GM9330@dhcp22.suse.cz> References: <20190731123201.13893-1-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731123201.13893-1-david@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed 31-07-19 14:32:01, David Hildenbrand wrote: > Let's document why we take the lock here. If we're going to overhaul > memory hotplug locking, we'll have to touch many places - this comment > will help to clairfy why it was added here. And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock. > Cc: Rafael J. Wysocki > Cc: Michal Hocko > Cc: Oscar Salvador > Cc: Andrew Morton > Signed-off-by: David Hildenbrand > --- > drivers/acpi/scan.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c > index cbc9d64b48dd..9193f1d46148 100644 > --- a/drivers/acpi/scan.c > +++ b/drivers/acpi/scan.c > @@ -2204,6 +2204,11 @@ int __init acpi_scan_init(void) > acpi_gpe_apply_masked_gpes(); > acpi_update_all_gpes(); > > + /* > + * We end up calling __add_memory(), which expects the > + * device_hotplug_lock to be held. Races with userspace and other > + * hotplug activities are not really possible - lock for consistency. > + */ > lock_device_hotplug(); > mutex_lock(&acpi_scan_lock); > > -- > 2.21.0 -- Michal Hocko SUSE Labs