linux-kernel - Re: [PATCH] signals: Avoid unnecessary taking of sighand->siglock

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Message-ID: <57E9915B.5000307@hpe.com>
Date:   Mon, 26 Sep 2016 17:21:31 -0400
From:   Waiman Long <waiman.long@....com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stas Sergeev <stsp@...t.ru>, <linux-kernel@...r.kernel.org>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH] signals: Avoid unnecessary taking of sighand->siglock

On 09/23/2016 05:32 AM, Oleg Nesterov wrote:
> On 09/22, Waiman Long wrote:
>> This patch is currently only active for 64-bit architectures.
> Why?
>
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2485,6 +2485,16 @@ void __set_current_blocked(const sigset_t *newset)
>>   {
>>   	struct task_struct *tsk = current;
>>
>> +	/*
>> +	 * In case the signal mask hasn't changed, we won't need to take
>> +	 * the lock. As the current blocked mask can be modified by other
>> +	 * CPUs,
> No, nobody else should modify current->blocked.

Thanks for the clarification. I am just not 100% sure about that. I will 
modify the comment.

>
> Yes, we need to cleanup the usage of force_sig_info(), and probably remove
> the "struct task_struct *t" argument.
>
>> we need to do an atomic read without lock. In other words,
>> +	 * this check will only be done on 64-bit systems.
>> +	 */
>> +#if _NSIG_WORDS == 1
>> +	if (READ_ONCE(tsk->blocked.sig[0]) == newset->sig[0])
>> +		return;
>> +#endif
> OK, agreed, but this should not depend on _NSIG_WORDS == 1 and
> READ_ONCE() looks confusing. It seems you need to add the new helper
> into include/linux/signal.h.

Yes, I will add a helper into signal.h and call it instead.

Cheers,
Longman