linux-kernel - Re: [PATCH v2 1/2] uprobes: Remove redundant spinlock in uprobe_deny

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Message-ID: <20240812120738.GC11656@redhat.com>
Date: Mon, 12 Aug 2024 14:07:39 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Liao Chang <liaochang1@...wei.com>
Cc: mhiramat@...nel.org, peterz@...radead.org, mingo@...hat.com,
	acme@...nel.org, namhyung@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, andrii@...nel.org, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2 1/2] uprobes: Remove redundant spinlock in
 uprobe_deny_signal()

On 08/09, Liao Chang wrote:
>
> Since clearing a bit in thread_info is an atomic operation, the spinlock
> is redundant and can be removed, reducing lock contention is good for
> performance.

My ack still stays, but let me add some notes...

sighand->siglock doesn't protect clear_bit() per se. It was used to not
break the "the state of TIF_SIGPENDING of every thread is stable with
sighand->siglock held" rule.

But we already have the lockless users of clear_thread_flag(TIF_SIGPENDING)
(some if not most of them look buggy), and afaics in this (very special)
case it should be fine.

Oleg.

> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Liao Chang <liaochang1@...wei.com>
> ---
>  kernel/events/uprobes.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 73cc47708679..76a51a1f51e2 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1979,9 +1979,7 @@ bool uprobe_deny_signal(void)
>  	WARN_ON_ONCE(utask->state != UTASK_SSTEP);
>  
>  	if (task_sigpending(t)) {
> -		spin_lock_irq(&t->sighand->siglock);
>  		clear_tsk_thread_flag(t, TIF_SIGPENDING);
> -		spin_unlock_irq(&t->sighand->siglock);
>  
>  		if (__fatal_signal_pending(t) || arch_uprobe_xol_was_trapped(t)) {
>  			utask->state = UTASK_SSTEP_TRAPPED;
> -- 
> 2.34.1
>