linux-kernel - Re: [PATCH] arm64/trap: fix broken ct->nmi_nesting when die() is called in a kthread

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Message-ID: <aD8TW7j3TwUsPLqJ@e129823.arm.com>
Date: Tue, 3 Jun 2025 16:23:07 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Will Deacon <will@...nel.org>, catalin.marinas@....com,
	geert@...ux-m68k.org, broonie@...nel.org, mcgrof@...nel.org,
	joey.gouly@....com, kristina.martsenko@....com, rppt@...nel.org,
	pcc@...gle.com, bigeasy@...utronix.de, ptosi@...gle.com,
	james.morse@....com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, ada.coupriediaz@....com
Subject: Re: [PATCH] arm64/trap: fix broken ct->nmi_nesting when die() is
 called in a kthread

Hi Mark,

> On Tue, Jun 03, 2025 at 12:14:18PM +0100, Yeoreum Yun wrote:
> > > On Mon, Jun 02, 2025 at 06:50:53PM +0100, Yeoreum Yun wrote:
> > > > So, what I think:
> > > >   1. arm64_enter_el1_dbg() should ct_nmi_enter() as it is.
> > > >   2. in bug_handler() while handling BUG_TYPE, add above ct_nmi_exit()
> > > >      conditional call.
> > > >   3. DAIF.D and DAIF.A handling.
> > >
> > > No, that is not safe. In step 2, calling ct_nmi_exit() would undo *all*
> > > of the ct_nmi_enter() logic, and may stop RCU from watching if the
> > > exception was entered from some intermediate/inconsistent state.
> >
> > Yes if call ct_nmi_enter() without condition.
> > But I imply with the condition check what I posted.
> > if CT_NESTING_IRQ_NONIDLE,
> > it wouldn't need call and that cpu can be watched by RCU.
>
> I am not keen on conditionally calling ct_nmi_exit(), and would strongly
> prefer to avoid that, regardless of where that lives in the flow.
>
> I suspect that it would be bettter to triage the interrupted context
> earlier, and rethink the way entry/exit works, but that's a much larger
> bit of work and will take more thinking.

Thanks for sharing your thought.
I'll think about it and let me raise it again after ada's patchset is
merged.


--
Sincerely,
Yeoreum Yun