linux-kernel - Re: [PATCH v13 09/14] unwind deferred: Use SRCU unwind_deferred_task

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Message-ID: <20250714135638.GC4105545@noisy.programming.kicks-ass.net>
Date: Mon, 14 Jul 2025 15:56:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: 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, Jul 07, 2025 at 09:22:48PM -0400, Steven Rostedt wrote:

> @@ -143,6 +144,7 @@ static void unwind_deferred_task_work(struct callback_head *head)
>  	struct unwind_stacktrace trace;
>  	struct unwind_work *work;
>  	u64 cookie;
> +	int idx;
>  
>  	if (WARN_ON_ONCE(!local_read(&info->pending)))
>  		return;
> @@ -161,13 +163,15 @@ static void unwind_deferred_task_work(struct callback_head *head)
>  
>  	cookie = info->id.id;
>  
> -	guard(mutex)(&callback_mutex);
> -	list_for_each_entry(work, &callbacks, list) {
> +	idx = srcu_read_lock(&unwind_srcu);
> +	list_for_each_entry_srcu(work, &callbacks, list,
> +				 srcu_read_lock_held(&unwind_srcu)) {
>  		if (test_bit(work->bit, &info->unwind_mask)) {
>  			work->func(work, &trace, cookie);
>  			clear_bit(work->bit, &info->unwind_mask);
>  		}
>  	}
> +	srcu_read_unlock(&unwind_srcu, idx);
>  }

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);
 	}