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Sender: Guenter Roeck <groeck7@gmail.com>
Subject: Re: [tip: sched/core] sched/core: Initialize the idle task with
 preemption disabled
To:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-tip-commits@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org
References: <20210512094636.2958515-1-valentin.schneider@arm.com>
 <162081815405.29796.14574924529325899839.tip-bot2@tip-bot2>
 <20210706194456.GA1823793@roeck-us.net> <87fswr6lqv.mognet@arm.com>
From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <0d792e69-8ba3-5658-16ea-c1090bffa410@roeck-us.net>
Date:   Tue, 6 Jul 2021 21:06:40 -0700
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On 7/6/21 4:55 PM, Valentin Schneider wrote:
> 
> Hi Guenter,
> 
> On 06/07/21 12:44, Guenter Roeck wrote:
>> This patch results in several messages similar to the following
>> when booting s390 images in qemu.
>>
>> [    1.690807] BUG: sleeping function called from invalid context at include/linux/percpu-rwsem.h:49
>> [    1.690925] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
>> [    1.691053] no locks held by swapper/0/1.
>> [    1.691310] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-11788-g79160a603bdb #1
>> [    1.691469] Hardware name: QEMU 2964 QEMU (KVM/Linux)
>> [    1.691612] Call Trace:
>> [    1.691718]  [<0000000000d98bb0>] show_stack+0x90/0xf8
>> [    1.692040]  [<0000000000da894c>] dump_stack_lvl+0x74/0xa8
>> [    1.692134]  [<0000000000187e52>] ___might_sleep+0x15a/0x170
>> [    1.692228]  [<000000000014f588>] cpus_read_lock+0x38/0xc0
>> [    1.692320]  [<0000000000182e8a>] smpboot_register_percpu_thread+0x2a/0x160
>> [    1.692412]  [<00000000014814b8>] cpuhp_threads_init+0x28/0x60
>> [    1.692505]  [<0000000001487a30>] smp_init+0x28/0x90
>> [    1.692597]  [<00000000014779a6>] kernel_init_freeable+0x1f6/0x270
>> [    1.692689]  [<0000000000db7466>] kernel_init+0x2e/0x160
>> [    1.692779]  [<0000000000103618>] __ret_from_fork+0x40/0x58
>> [    1.692870]  [<0000000000dc6e12>] ret_from_fork+0xa/0x30
>>
>> Reverting this patch fixes the problem.
>> Bisect log is attached.
>>
>> Guenter
>>
> 
> 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?
> 

My configuration doesn't have CONFIG_PREEMPT enabled.

Guenter