linux-kernel - Re: [RFC] change non-atomic bitops method

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 2 Feb 2015 15:29:09 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>
Cc:	"'arnd@...db.de'" <arnd@...db.de>,
	"'linux-arch@...r.kernel.org'" <linux-arch@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'linux@....linux.org.uk'" <linux@....linux.org.uk>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] change non-atomic bitops method

On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin" <Yalin.Wang@...ymobile.com> wrote:

> This patch change non-atomic bitops,
> add a if() condition to test it, before set/clear the bit.
> so that we don't need dirty the cache line, if this bit
> have been set or clear. On SMP system, dirty cache line will
> need invalidate other processors cache line, this will have
> some impact on SMP systems.
> 
> --- a/include/asm-generic/bitops/non-atomic.h
> +++ b/include/asm-generic/bitops/non-atomic.h
> @@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile unsigned long *addr)
>  	unsigned long mask = BIT_MASK(nr);
>  	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
>  
> -	*p  |= mask;
> +	if ((*p & mask) == 0)
> +		*p  |= mask;
> +
>  }

hm, maybe.

It will speed up set_bit on an already-set bit.  But it will slow down
set_bit on a not-set bit.  And the latter case is presumably much, much
more common.

How do we know the patch is a net performance gain?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/