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

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Message-ID: <20200206163844.GA432041@zx2c4.com>
Date:   Thu, 6 Feb 2020 17:38:44 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     eric.dumazet@...il.com
Cc:     cai@....pw, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()

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?

Jason