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

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Message-ID: <20160923093241.GA13792@redhat.com>
Date:   Fri, 23 Sep 2016 11:32:42 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Waiman Long <Waiman.Long@....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 09/22, 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.

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.

Oleg.