Re: [mm] 408579cd62: WARNING:suspicious_RCU_usage

[Conor Dooley <conor@kernel.org>]

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On Mon, Jul 03, 2023 at 10:07:28AM -0700, Linus Torvalds wrote:
> On Mon, 3 Jul 2023 at 10:00, Conor Dooley <conor@kernel.org> 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.

Ah, I probably should've known from the
         RCU used illegally from offline CPU!
that it was different.

> 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.

Taking the rationale & potential problems out of the equation, that
code movement does suppress the complaints from rcu/maple tree,
thanks.

Cheers,
Conor.

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