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From: Herbert Xu <herbert@gondor.apana.org.au>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: eric.dumazet@gmail.com, cai@lca.pw, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()
Date: Mon, 17 Feb 2020 11:24:58 +0800	[thread overview]
Message-ID: <20200217032458.kwatitz3pvxeb25w@gondor.apana.org.au> (raw)
In-Reply-To: <20200206163844.GA432041@zx2c4.com>

Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Hi Eric,
> 
> On Tue, Feb 04, 2020 at 01:40:29PM -0500, Qian Cai wrote:
>> -     list->qlen--;
>> +     WRITE_ONCE(list->qlen, list->qlen - 1);
> 
> Sorry I'm a bit late to the party here, but this immediately jumped out.
> This generates worse code with a bigger race in some sense:
> 
> list->qlen-- is:
> 
>   0:   83 6f 10 01             subl   $0x1,0x10(%rdi)
> 
> whereas WRITE_ONCE(list->qlen, list->qlen - 1) is:
> 
>   0:   8b 47 10                mov    0x10(%rdi),%eax
>   3:   83 e8 01                sub    $0x1,%eax
>   6:   89 47 10                mov    %eax,0x10(%rdi)
> 
> Are you sure that's what we want?

Fixing these KCSAN warnings is actively making the kernel worse.

Why are we still doing this?

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

  parent reply	other threads:[~2020-02-17  3:25 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-04 18:40 [PATCH v3] skbuff: fix a data race in skb_queue_len() Qian Cai
2020-02-06 12:59 ` David Miller
2020-02-06 16:38 ` Jason A. Donenfeld
2020-02-06 17:10   ` Eric Dumazet
2020-02-06 18:12     ` Jason A. Donenfeld
2020-02-06 18:22       ` Eric Dumazet
2020-02-06 18:43         ` Jason A. Donenfeld
2020-02-06 19:29           ` Marco Elver
2020-02-06 21:55           ` Jason A. Donenfeld
2020-02-07 10:35     ` Marco Elver
2020-02-17  3:24   ` Herbert Xu [this message]
2020-02-17  7:39     ` Jason A. Donenfeld
2020-02-17 10:20       ` Herbert Xu

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