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

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Message-ID: <20250122122821.GN7145@noisy.programming.kicks-ass.net>
Date: Wed, 22 Jan 2025 13:28:21 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.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 Tue, Jan 21, 2025 at 06:30:53PM -0800, Josh Poimboeuf wrote:
> It's possible for irq_work_queue() to fail if the work has already been
> claimed.  That can happen if a TWA_NMI_CURRENT task work is requested
> before a previous TWA_NMI_CURRENT IRQ work on the same CPU has gotten a
> chance to run.

I'm confused, if it fails then it's already pending, and we'll get the
notification already. You can still add the work.

> The error has to be checked before the write to task->task_works.  Also
> the try_cmpxchg() loop isn't needed in NMI context.  The TWA_NMI_CURRENT
> case really is special, keep things simple by keeping its code all
> together in one place.

NMIs can nest, consider #DB (which is NMI like) doing task_work_add()
and getting interrupted with NMI doing the same.


Might all this be fallout from trying to fix that schedule() bug from
the next patch, because as is, I don't see it.