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Message-ID: <20250714102140.4886afa0@batman.local.home>
Date: Mon, 14 Jul 2025 10:21:40 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.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>,
 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 v13 09/14] unwind deferred: Use SRCU
 unwind_deferred_task_work()

On Mon, 14 Jul 2025 15:56:38 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> Please; something like so:
> 
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -524,4 +524,9 @@ DEFINE_LOCK_GUARD_1(srcu, struct srcu_st
>  		    srcu_read_unlock(_T->lock, _T->idx),
>  		    int idx)
>  
> +DEFINE_LOCK_GUARD_1(srcu_lite, struct srcu_struct,
> +		    _T->idx = srcu_read_lock_lite(_T->lock),
> +		    srcu_read_unlock_lite(_T->lock, _T->idx),
> +		    int idx)
> +
>  #endif
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -165,7 +165,7 @@ static void unwind_deferred_task_work(st
>  
>  	cookie = info->id.id;
>  
> -	guard(mutex)(&callback_mutex);
> +	guard(srcu_lite)(&unwind_srcu);
>  	list_for_each_entry(work, &callbacks, list) {
>  		work->func(work, &trace, cookie);
>  	}

I think I rather have a scoped_guard() here. One thing that bothers me
about the guard() logic is that it could easily start to "leak"
protection. That is, the unwind_srcu is only needed for walking the
list. The reason I chose to open code the protection, is because I
wanted to distinctly denote where the end of the protection was.

-- Steve

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