linux-kernel - Re: [tip: sched/core] sched/core: Initialize the idle task with preemption disabled

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:   Wed, 07 Jul 2021 13:11:59 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org,
        linux-tip-commits@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Subject: Re: [tip: sched/core] sched/core: Initialize the idle task with preemption disabled

On 07/07/21 14:03, Frederic Weisbecker wrote:
> On Wed, Jul 07, 2021 at 12:55:20AM +0100, Valentin Schneider wrote:
>> Thanks for the report.
>>
>> So somehow the init task ends up with a non-zero preempt_count()? Per
>> FORK_PREEMPT_COUNT we should exit __ret_from_fork() with a zero count, are
>> you hitting the WARN_ONCE() in finish_task_switch()?
>>
>> Does CONFIG_DEBUG_PREEMPT=y yield anything interesting?
>>
>> I can't make sense of this right now, but it's a bit late :) I'll grab some
>> toolchain+qemu tomorrow and go poke at it (and while at it I need to do the
>> same with powerpc).
>
> One possible issue is that s390's init_idle_preempt_count() doesn't apply on the
> target idle task but on the _current_ CPU. And since smp_init() ->
> idle_threads_init() is actually called remotely, we are overwriting the current
> CPU preempt_count() instead of the target one.

Indeed, this becomes quite obvious when tracing the preemption count
changes. This also means that s390 relied on the idle_thread_get() from the
hotplug machinery to properly setup the preempt count, rather than
init_idle_preempt_count() - which is quite yuck.

I'll write a patch for that and likely one for powerpc.