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

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Message-ID: <20250717082526.7173106a@gandalf.local.home>
Date: Thu, 17 Jul 2025 08:25:26 -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 Wed, 16 Jul 2025 21:43:47 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > +DEFINE_LOCK_GUARD_1(srcu_lite, struct srcu_struct,  
> 
> You need srcu_fast because srcu_lite is being removed.  They are quite
> similar, but srcu_fast is faster and is NMI-safe.  (This last might or
> might not matter here.)
> 
> See https://lore.kernel.org/all/20250716225418.3014815-3-paulmck@kernel.org/
> for a srcu_fast_notrace, so something like this:

Yeah, I already saw that patch.

> 
> DEFINE_LOCK_GUARD_1(srcu_fast, struct srcu_struct,
> 		    _T->scp = srcu_read_lock_fast(_T->lock),
> 		    srcu_read_unlock_fast(_T->lock, _T->scp),
> 		    struct srcu_ctr __percpu *scp)
> 
> Other than that, it looks plausible.

Using srcu_lite or srcu_fast is an optimization here. And since I saw you
adding the guard for srcu_fast in that other thread, I'll just use normal
SRCU here for this series, and in the future we could convert it over to
srcu_fast.

Thanks!

-- Steve