linux-kernel - Re: [PATCH 4.19 599/639] net: avoid possible false sharing in sk_leave_memory

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: Windows password security audit tool. GUI, reports in PDF.

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

Date:   Sun, 26 Jan 2020 18:15:05 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 599/639] net: avoid possible false sharing in
 sk_leave_memory_pressure()

On Fri 2020-01-24 10:32:49, Greg Kroah-Hartman wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> [ Upstream commit 503978aca46124cd714703e180b9c8292ba50ba7 ]
> 
> As mentioned in https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance
> a C compiler can legally transform :
> 
> if (memory_pressure && *memory_pressure)
>         *memory_pressure = 0;
> 
> to :
> 
> if (memory_pressure)
>         *memory_pressure = 0;

Well, C compiler can do a lot of stuff, and we rely on C compiler
being "sane" -- that is gcc.

Even if compiler did the transformation, that will only result in
slightly slower performance, right?

Is there any evidence this is problem in practice? Should this be in
stable?

Best regards,
								Pavel


> +++ b/net/core/sock.c
> @@ -2179,8 +2179,8 @@ static void sk_leave_memory_pressure(struct sock *sk)
>  	} else {
>  		unsigned long *memory_pressure = sk->sk_prot->memory_pressure;
>  
> -		if (memory_pressure && *memory_pressure)
> -			*memory_pressure = 0;
> +		if (memory_pressure && READ_ONCE(*memory_pressure))
> +			WRITE_ONCE(*memory_pressure, 0);
>  	}
>  }



-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Download attachment "signature.asc" of type "application/pgp-signature" (196 bytes)