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

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Message-ID: <2971107e-75e7-8438-c858-b95202d7b5ea@huawei.com>
Date: Tue, 13 Aug 2024 20:30:32 +0800
From: "Liao, Chang" <liaochang1@...wei.com>
To: Oleg Nesterov <oleg@...hat.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()



在 2024/8/12 20:07, Oleg Nesterov 写道:
> 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, your explaination is more accurate. So I will reword the commit log and
quote some of your note like this:

  Since we already have the lockless user of clear_thread_flag(TIF_SIGPENDING).
  And for uprobe singlestep case, it doesn't break the rule of "the state of
  TIF_SIGPENDING of every thread is stable with sighand->siglock held". So
  removing sighand->siglock to reduce contention for better performance.

> 
> 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
>>
> 
> 

-- 
BR
Liao, Chang