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

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Message-ID: <20250422121454.523757af@gandalf.local.home>
Date: Tue, 22 Apr 2025 12:14:54 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.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


/me is resurrecting the sframe work

On Wed, 22 Jan 2025 13:28:21 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.
> 

I was looking at this patch and was thinking that the trycmpxchg() is still
for the race against other CPUs.

Another CPU can add a task_work for this task, right?

If so, then even though the NMI adding the task work can't be interrupted
(considering there's no #DB and such), it still can clobber an update done
for this tasks task_work from another CPU.

Is this patch still even needed?

-- Steve