linux-kernel - Re: [PATCH] Fix the race between smp_call

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]

Message-ID: <1332250867.18960.423.camel@twins>
Date:	Tue, 20 Mar 2012 14:41:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] Fix the race between smp_call_function and CPU booting

On Tue, 2012-03-20 at 18:44 +0530, Srivatsa S. Bhat wrote:
> 
> 
> I don't think this patch would change anything, atleast it wouldn't get
> rid of the warning that Liu reported. Because, he is running his stress
> tests on a machine which has only 2 CPUs. So effectively, we are hotplugging
> only CPU1 (since CPU0 can't be taken offline, on Intel boxes).
> 
> Also, CPU1 is removed from cpu_active_mask during CPU_DOWN_PREPARE time itself,
> and migrate_tasks() comes much later (during CPU_DYING). And in any case,
> dest_cpu will never be CPU1, because it is the CPU that is going down. So it
> *has* to be CPU0 anyway.
> 
> So, I don't think changes to select_fallback_rq() to make it more careful is
> going to make any difference in the particular scenario that Liu is testing.
> 
> That said, even I don't know what the root cause of the warning is.. :-(

Its a race in cpu-up, we set active before online, when we do a wakeup
select_task_rq() will see !cpu_online(), we then call
select_fallback_rq() to compute a new cpu, select_fallback_rq() computes
a new cpu against cpu_active (which is set) and can thus return cpu 1,
even though it is still offline.

So we queue the task on cpu 1 and send a reschedule ipi, at which point
we'll get the reported warning.

My change modifies select_fallback_rq() to require online && active.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/