linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.DEB.2.20.1702091048330.24346@east.gentwo.org>
Date:   Thu, 9 Feb 2017 11:22:49 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

On Thu, 9 Feb 2017, Thomas Gleixner wrote:

> You are just not getting it, really.
>
> The problem is that this for_each_online_cpu() is racy against a concurrent
> hot unplug and therefor can queue stuff for a not longer online cpu. That's
> what the mm folks tried to avoid by preventing a CPU hotplug operation
> before entering that loop.

With a stop machine action it is NOT racy because the machine goes into a
special kernel state that guarantees that key operating system structures
are not touched. See mm/page_alloc.c's use of that characteristic to build
zonelists. Thus it cannot be executing for_each_online_cpu and related
tasks (unless one does not disable preempt .... but that is a given if a
spinlock has been taken)..

> > Lets get rid of get_online_cpus() etc.
>
> And that solves what?

It gets rid of future issues with serialization in paths were we need to
lock and still do for_each_online_cpu().

> Can you please start to understand the scope of the whole hotplug machinery
> including the requirements for get_online_cpus() before you waste
> everybodys time with your uninformed and halfbaken proposals?

Its an obvious solution to the issues that have arisen multiple times with
get_online_cpus() within the slab allocators. The hotplug machinery should
make things as easy as possible for other people and having these
get_online_cpus() everywhere does complicate things.