linux-kernel - Re: [PATCH v3] skbuff: fix a data race in skb_queue

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Date:   Mon, 17 Feb 2020 11:24:58 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     eric.dumazet@...il.com, cai@....pw, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()

Jason A. Donenfeld <Jason@...c4.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@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt