From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752863AbaBMTx2 (ORCPT ); Thu, 13 Feb 2014 14:53:28 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:34708 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751397AbaBMTx0 (ORCPT ); Thu, 13 Feb 2014 14:53:26 -0500 Date: Thu, 13 Feb 2014 11:53:25 -0800 From: Andrew Morton To: Vladimir Davydov Cc: , , , , Subject: Re: [PATCH 1/2] kobject: don't block for each kobject_uevent Message-Id: <20140213115325.0afcef6b57b63a2ce407c76b@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 9 Feb 2014 14:56:15 +0400 Vladimir Davydov wrote: > Currently kobject_uevent has somewhat unpredictable semantics. The point > is, since it may call a usermode helper and wait for it to execute > (UMH_WAIT_EXEC), it is impossible to say for sure what lock dependencies > it will introduce for the caller - strictly speaking it depends on what > fs the binary is located on and the set of locks fork may take. There > are quite a few kobject_uevent's users that do not take this into > account and call it with various mutexes taken, e.g. rtnl_mutex, > net_mutex, which might potentially lead to a deadlock. > > Since there is actually no reason to wait for the usermode helper to > execute there, let's make kobject_uevent start the helper asynchronously > with the aid of the UMH_NO_WAIT flag. > > Personally, I'm interested in this, because I really want kobject_uevent > to be called under the slab_mutex in the slub implementation as it used > to be some time ago, because it greatly simplifies synchronization and > automatically fixes a kmemcg-related race. However, there was a deadlock > detected on an attempt to call kobject_uevent under the slab_mutex (see > https://lkml.org/lkml/2012/1/14/45), which was reported to be fixed by > releasing the slab_mutex for kobject_uevent. Unfortunately, there was no > information about who exactly blocked on the slab_mutex causing the > usermode helper to stall, neither have I managed to find this out or > reproduce the issue. > > BTW, this is not the first attempt to make kobject_uevent use > UMH_NO_WAIT. Previous one was made by commit f520360d93c, but it was > wrong (it passed arguments allocated on stack to async thread) so it was > reverted (commit 05f54c13cd0c). It targeted on speeding up the boot > process though. Am not a huge fan of this patch. My test box gets an early oops in initcalls ->... ->register_pernet_operations ->rtnetlink_net_init ->__netlink_kernel_create ->sock_create_lite ->sock_alloc ->new_inode_pseudo ->alloc_inode+0xe I expect that sock_mnt->mnt_sb is null. Or perhaps sb->s_op. Perhaps sockfs hasn't mounted yet. The oops doesn't happen on mainline - it only happens on linux-next. So there may be some interaction there, but it may only be timing related. config: http://ozlabs.org/~akpm/stuff/config-akpm2