linux-kernel - Re: [PATCH v14 09/12] unwind deferred: Use SRCU unwind_deferred_task

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Message-ID: <20250717123835.21c8aa89@batman.local.home>
Date: Thu, 17 Jul 2025 12:38:35 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org,
 Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Thomas
 Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, Indu
 Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, Beau
 Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton
 <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, Florian Weimer
 <fweimer@...hat.com>, Sam James <sam@...too.org>
Subject: Re: [PATCH v14 09/12] unwind deferred: Use SRCU
 unwind_deferred_task_work()

On Thu, 17 Jul 2025 09:27:34 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > Two, I'm still grasping at the concept of srcu_fast (and srcu_lite for
> > that matter), where I rather be slow and safe than optimize and be
> > unsafe. The code where this is used may be faulting in user space
> > memory, so it doesn't need the micro-optimizations now.  
> 
> Straight-up SRCU and guard(srcu), then?  Both are already in mainline.
> 
> Or are those read-side smp_mb() calls a no-go for this code?

As I stated, the read-side is likely going to be faulting in user space
memory. I don't think one or two smp_mb() will really make much of a
difference ;-)

It's not urgent. If it can be switched to srcu_fast, we can do it later.

Thanks,

-- Steve