linux-kernel - Re: [PATCH v4 01/39] task_work: Fix TWA_NMI

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Message-ID: <20250123171509.3xmrbrbmu3tsll7w@jpoimboe>
Date: Thu, 23 Jan 2025 09:15:09 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 01/39] task_work: Fix TWA_NMI_CURRENT error handling

On Thu, Jan 23, 2025 at 09:14:03AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 22, 2025 at 12:47:20PM -0800, Josh Poimboeuf wrote:
> > What exactly do you mean by "NMI like"?  Is it because a #DB might be
> > basically running in NMI context, if the NMI hit a breakpoint?
> 
> No, #DB, #BP and such are considered NMI (and will have in_nmi() true)
> because they can trigger anywhere, including sections where IRQs are
> disabled.

So:

  - while exceptions are technically not NMI, they're "NMI" because they
    can occur in NMI or IRQ-disabled regions

  - such "NMI" exceptions can be preempted by NMIs and "NMIs"

  - NMIs can be preempted by "NMIs" but not NMIs (except in entry code!)

... did I get all that right?  Not subtle at all!

I feel like in_nmi() needs a comment explaining all that nonobviousness.

-- 
Josh