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SubjectRe: [tip: sched/core] sched/core: Initialize the idle task with preemption disabled
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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?

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).

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