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: Windows password security audit tool. GUI, reports in PDF.

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

Date:   Thu, 9 Feb 2017 12:42:44 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christoph Lameter <cl@...ux.com>
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 Wed, 8 Feb 2017, Christoph Lameter wrote:
> On Wed, 8 Feb 2017, Thomas Gleixner wrote:
> 
> > There is a world outside yours. Hotplug is actually used frequently for
> > power purposes in some scenarios.
> 
> The usual case does not inolve hotplug.

We do not care about your definition of "usual". The kernel serves _ALL_
use cases.

> > It will improve nothing. The stop machine context is extremly limited and
> > you cannot do complex things there at all. Not to talk about the inability
> > of taking a simple mutex which would immediately deadlock the machine.
> 
> You do not need to do complex things. Basically flipping some cpu mask
> bits will do it. stop machine ensures that code is not
> executing on the processors when the bits are flipped. That will ensure
> that there is no need to do any get_online_cpu() nastiness in critical VM
> paths since we are guaranteed not to be executing them.

And how does that solve the problem at hand? Not at all:

CPU 0	     	  	    CPU 1

for_each_online_cpu(cpu)
 ==> cpu = 1
			    stop_machine()
			    set_cpu_online(1, false)
 queue_work(cpu1)

Thanks,

	tglx