Re: [PATCH] init/main: Clear boot task idle flag

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Liam,

On Wed, Sep 13, 2023 at 2:57 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> Initial booting is setting the task flag to idle (PF_IDLE) by the call
> path sched_init() -> init_idle().  Having the task idle and calling
> call_rcu() in kernel/rcu/tiny.c means that TIF_NEED_RESCHED will be
> set.  Subsequent calls to any cond_resched() will enable IRQs,
> potentially earlier than the IRQ setup has completed.  Recent changes
> have caused just this scenario and IRQs have been enabled early.
>
> This causes a warning later in start_kernel() as interrupts are enabled
> before they are fully set up.
>
> Fix this issue by clearing the PF_IDLE flag on return from sched_init()
> and restore the flag in rest_init().  Although the boot task was marked
> as idle since (at least) d80e4fda576d, I am not sure that it is wrong to
> do so.  The forced context-switch on idle task was introduced in the
> tiny_rcu update, so I'm going to claim this fixes 5f6130fa52ee.
>
> Link: https://lore.kernel.org/linux-mm/87v8cv22jh.fsf@mail.lhotse/
> Link: https://lore.kernel.org/linux-mm/CAMuHMdWpvpWoDa=Ox-do92czYRvkok6_x6pYUH+ZouMcJbXy+Q@mail.gmail.com/
> Fixes: 5f6130fa52ee ("tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task")
> Cc: stable@vger.kernel.org
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peng Zhang <zhangpeng.00@bytedance.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>

Thanks for your patch!

This fixes the

      WARNING: CPU: 0 PID: 0 at init/main.c:992 start_kernel+0x2f0/0x480

I was seeing during boot on Renesas RZ/A1 and RZ/A2 since commit
cfeb6ae8bcb96ccf ("maple_tree: disable mas_wr_append() when other
readers are possible") in v6.5.

However, this causes lots of new warnings on e.g. R-Car M2-W, which
wasn't affected before, like:

+BUG: sleeping function called from invalid context at
kernel/sched/completion.c:111
+in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
+preempt_count: 0, expected: 0
+no locks held by swapper/0/1.
+CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W
6.6.0-rc1-koelsch-02359-g9a5bed6a8bd8 #1864
+Hardware name: Generic R-Car Gen2 (Flattened Device Tree)
+ unwind_backtrace from show_stack+0x10/0x14
+ show_stack from dump_stack_lvl+0x68/0x90
+ dump_stack_lvl from __might_resched+0x1b0/0x22c
+ __might_resched from __wait_for_common+0x38/0x170
+ __wait_for_common from kernel_init+0x10/0x130
+ kernel_init from ret_from_fork+0x14/0x30
+Exception stack(0xf0815fb0 to 0xf0815ff8)
+5fa0:                                     00000000 00000000 00000000 00000000
+5fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
+5fe0: 00000000 00000000 00000000 00000000 00000013 00000000

and

+BUG: sleeping function called from invalid context at
kernel/locking/mutex.c:580
+in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: systemd
+preempt_count: 0, expected: 0
+no locks held by systemd/1.
+CPU: 1 PID: 1 Comm: systemd Tainted: G        W        N
6.6.0-rc1-koelsch-02359-g9a5bed6a8bd8 #1864
+Hardware name: Generic R-Car Gen2 (Flattened Device Tree)
+ unwind_backtrace from show_stack+0x10/0x14
+ show_stack from dump_stack_lvl+0x68/0x90
+ dump_stack_lvl from __might_resched+0x1b0/0x22c
+ __might_resched from __mutex_lock+0x34/0x3a8
+ __mutex_lock from mutex_lock_nested+0x18/0x20
+ mutex_lock_nested from do_epoll_wait+0x220/0x5ac
+ do_epoll_wait from sys_epoll_wait+0x44/0x6c
+ sys_epoll_wait from ret_fast_syscall+0x0/0x1c
+Exception stack(0xf0815fa8 to 0xf0815ff0)
+5fa0:                   beff75e0 00000004 00000004 beff75e0 0000003b ffffffff
+5fc0: beff75e0 00000004 ffffffff 000000fc 00000000 ffffffff 0000003b 00000004
+5fe0: 000000fc beff75b8 b6ef8511 b6e75786

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds