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

[Guenter Roeck <linux@roeck-us.net>]

Hi,

On Wed, May 12, 2021 at 11:15:54AM -0000, tip-bot2 for Valentin Schneider wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674
> Gitweb:        https://git.kernel.org/tip/f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674
> Author:        Valentin Schneider <valentin.schneider@arm.com>
> AuthorDate:    Wed, 12 May 2021 10:46:36 +01:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Wed, 12 May 2021 13:01:45 +02:00
> 
> sched/core: Initialize the idle task with preemption disabled
> 
> As pointed out by commit
> 
>   de9b8f5dcbd9 ("sched: Fix crash trying to dequeue/enqueue the idle thread")
> 
> init_idle() can and will be invoked more than once on the same idle
> task. At boot time, it is invoked for the boot CPU thread by
> sched_init(). Then smp_init() creates the threads for all the secondary
> CPUs and invokes init_idle() on them.
> 
> As the hotplug machinery brings the secondaries to life, it will issue
> calls to idle_thread_get(), which itself invokes init_idle() yet again.
> In this case it's invoked twice more per secondary: at _cpu_up(), and at
> bringup_cpu().
> 
> Given smp_init() already initializes the idle tasks for all *possible*
> CPUs, no further initialization should be required. Now, removing
> init_idle() from idle_thread_get() exposes some interesting expectations
> with regards to the idle task's preempt_count: the secondary startup always
> issues a preempt_disable(), requiring some reset of the preempt count to 0
> between hot-unplug and hotplug, which is currently served by
> idle_thread_get() -> idle_init().
> 
> Given the idle task is supposed to have preemption disabled once and never
> see it re-enabled, it seems that what we actually want is to initialize its
> preempt_count to PREEMPT_DISABLED and leave it there. Do that, and remove
> init_idle() from idle_thread_get().
> 
> Secondary startups were patched via coccinelle:
> 
>   @begone@
>   @@
> 
>   -preempt_disable();
>   ...
>   cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Acked-by: Peter Zijlstra <peterz@infradead.org>
> Link: https://lore.kernel.org/r/20210512094636.2958515-1-valentin.schneider@arm.com

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

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# bad: [f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674] sched/core: Initialize the idle task with preemption disabled
git bisect bad f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674
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# first bad commit: [f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674] sched/core: Initialize the idle task with preemption disabled