linux-kernel - Re: [PATCH v2] net/core: Fix BUG to BUG

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Date:   Mon, 9 Oct 2017 23:15:40 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Tim Hansen <devtimhansen@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "willemb@...gle.com" <willemb@...gle.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "soheil@...gle.com" <soheil@...gle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "elena.reshetova@...el.com" <elena.reshetova@...el.com>,
        "tom@...ntonium.net" <tom@...ntonium.net>,
        "Jason@...c4.com" <Jason@...c4.com>, "fw@...len.de" <fw@...len.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] net/core: Fix BUG to BUG_ON conditionals.

On Mon, Oct 09, 2017 at 04:06:20PM -0700, Alexei Starovoitov wrote:
>On Mon, Oct 09, 2017 at 08:26:34PM +0000, Levin, Alexander (Sasha Levin) wrote:
>> On Mon, Oct 09, 2017 at 10:15:42AM -0700, Alexei Starovoitov wrote:
>> >On Mon, Oct 09, 2017 at 11:37:59AM -0400, Tim Hansen wrote:
>> >> Fix BUG() calls to use BUG_ON(conditional) macros.
>> >>
>> >> This was found using make coccicheck M=net/core on linux next
>> >> tag next-2017092
>> >>
>> >> Signed-off-by: Tim Hansen <devtimhansen@...il.com>
>> >> ---
>> >>  net/core/skbuff.c | 15 ++++++---------
>> >>  1 file changed, 6 insertions(+), 9 deletions(-)
>> >>
>> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> >> index d98c2e3ce2bf..34ce4c1a0f3c 100644
>> >> --- a/net/core/skbuff.c
>> >> +++ b/net/core/skbuff.c
>> >> @@ -1350,8 +1350,7 @@ struct sk_buff *skb_copy(const struct sk_buff *skb, gfp_t gfp_mask)
>> >>  	/* Set the tail pointer and length */
>> >>  	skb_put(n, skb->len);
>> >>
>> >> -	if (skb_copy_bits(skb, -headerlen, n->head, headerlen + skb->len))
>> >> -		BUG();
>> >> +	BUG_ON(skb_copy_bits(skb, -headerlen, n->head, headerlen + skb->len));
>> >
>> >I'm concerned with this change.
>> >1. Calling non-trivial bit of code inside the macro is a poor coding style (imo)
>> >2. BUG_ON != BUG. Some archs like mips and ppc have HAVE_ARCH_BUG_ON and implementation
>> >of BUG and BUG_ON look quite different.
>>
>> For these archs, wouldn't it then be more efficient to use BUG_ON rather than BUG()?
>
>why more efficient? any data to prove that?

Just guessing.

Either way, is there a particular reason for not using BUG_ON() here
besides that it's implementation is "quite different"?

>I'm pointing that the change is not equivalent and
>this code has been around forever (pre-git days), so I see
>no reason to risk changing it.

Do you know that BUG_ON() is broken on any archs?

If not, "this code has been around forever" is really not an excuse to
not touch code.

If BUG_ON() behavior is broken somewhere, then it needs to get fixed.

-- 

Thanks,
Sasha