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Date: Tue, 22 Apr 2025 12:14:54 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>, x86@kernel.org, Ingo Molnar
 <mingo@kernel.org>, Arnaldo Carvalho de Melo <acme@kernel.org>,
 linux-kernel@vger.kernel.org, Indu Bhagat <indu.bhagat@oracle.com>, Mark
 Rutland <mark.rutland@arm.com>, Alexander Shishkin
 <alexander.shishkin@linux.intel.com>, Jiri Olsa <jolsa@kernel.org>,
 Namhyung Kim <namhyung@kernel.org>, Ian Rogers <irogers@google.com>, Adrian
 Hunter <adrian.hunter@intel.com>, linux-perf-users@vger.kernel.org, Mark
 Brown <broonie@kernel.org>, linux-toolchains@vger.kernel.org, Jordan Rome
 <jordalgo@meta.com>, Sam James <sam@gentoo.org>,
 linux-trace-kernel@vger.kernel.org, Andrii Nakryiko
 <andrii.nakryiko@gmail.com>, Jens Remus <jremus@linux.ibm.com>, Mathieu
 Desnoyers <mathieu.desnoyers@efficios.com>, Florian Weimer
 <fweimer@redhat.com>, Andy Lutomirski <luto@kernel.org>, Masami Hiramatsu
 <mhiramat@kernel.org>, Weinan Liu <wnliu@google.com>
Subject: Re: [PATCH v4 01/39] task_work: Fix TWA_NMI_CURRENT error handling
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/me is resurrecting the sframe work

On Wed, 22 Jan 2025 13:28:21 +0100
Peter Zijlstra <peterz@infradead.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