linux-kernel - Re: [PATCH v2] signals: Avoid unnecessary taking of sighand->siglock

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Message-ID: <20160926162831.GB6782@redhat.com>
Date:   Mon, 26 Sep 2016 18:28:31 +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 v2] signals: Avoid unnecessary taking of sighand->siglock

On 09/23, Waiman Long wrote:
>
>  
> +	/*
> +	 * In case the signal mask hasn't changed, we won't need to take
> +	 * the lock. The current blocked mask can be modified by other CPUs.
> +	 * To be safe, we need to do an atomic read without lock. As a result,
> +	 * this check will only be done on 64-bit architectures.
> +	 */
> +	if ((_NSIG_WORDS == 1) &&
> +	    (READ_ONCE(tsk->blocked.sig[0]) == newset->sig[0]))
> +		return;

so in case you missed my reply to V1, I still think that the comment is wrong
and you should drop the _NSIG_WORDS check.

Oleg.