Re: [mm] 408579cd62: WARNING:suspicious_RCU_usage

[Linus Torvalds]

On Mon, 3 Jul 2023 at 10:00, Conor Dooley <conor@xxxxxxxxxx> wrote:
>
> I'm not entirely sure if it is related, as stuff in the guts of mm like
> this is beyond me, but I've been seeing similar warnings on RISC-V.

No, that RISC-V warning is also about bad RCU usage, but that's a
different thing.

>         RCU used illegally from offline CPU!
>         rcu_scheduler_active = 1, debug_locks = 1
>         1 lock held by swapper/1/0:
>          #0: ffffffff8169ceb0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x0/0x32
>
>         stack backtrace:
>         CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.4.0-10173-ga901a3568fd2 #1
>         Hardware name: riscv-virtio,qemu (DT)
>         Call Trace:
>         [<ffffffff80006a20>] show_stack+0x2c/0x38
>         [<ffffffff80af3ee0>] dump_stack_lvl+0x5e/0x80
>         [<ffffffff80af3f16>] dump_stack+0x14/0x1c
>         [<ffffffff80083ff0>] lockdep_rcu_suspicious+0x19e/0x232
>         [<ffffffff80ad4802>] mtree_load+0x18a/0x3b6
>         [<ffffffff80091632>] __irq_get_desc_lock+0x2c/0x82
>         [<ffffffff80094722>] enable_percpu_irq+0x36/0x9e
>         [<ffffffff800087d4>] riscv_ipi_enable+0x32/0x4e
>         [<ffffffff80008692>] smp_callin+0x24/0x66

This is also triggering on the maple tree sanity checks, but it' sa
different maple tree, and a different code sequence.

And a different case of suspicious RCU usage - not a lack of locking,
but simply using RCU before marking the CPU online.

I suspect the riscv_ipi_enable() in the RISC-V version of smp_callin()
needs to be moved down to below the

        set_cpu_online(curr_cpuid, 1);

or was there some reason why it needed to be done quite _that_ early
in commit 832f15f42646 ("RISC-V: Treat IPIs as normal Linux IRQs")?

Added guilty parties to the cc.

               Linus