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From: Vipul Ashri <vipul.ashri@oracle.com>
To: Edward Makarov <makarov@kraftway.ru>,
	Andrew Rybchenko <arybchenko@solarflare.com>,
	dev@dpdk.org
Cc: chenbo.xia@intel.com, zhihong.wang@intel.com,
	maxime.coquelin@redhat.com,  stable@dpdk.org
Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in helper macro
Date: Fri, 18 Sep 2020 10:46:44 +0000 (UTC)	[thread overview]
Message-ID: <1b510ced-ba3a-4bf9-8d33-204073cfedba@default> (raw)
In-Reply-To: <f2538056-20ba-d957-2f12-904c7702047a@kraftway.ru>

Hi Edward / Andrew

I like your suggestions and applied with [v5] net/virtio: fix wrong variable assignment in helper macro
V4 had a extra line typos., V5 is tested compiled and pushed carefully Thanks!

Regards
Vipul

-----Original Message-----
From: Edward Makarov [mailto:makarov@kraftway.ru] 
Sent: Sunday, 30 August, 2020 3:48
To: Andrew Rybchenko <arybchenko@solarflare.com>; Vipul Ashri <vipul.ashri@oracle.com>; dev@dpdk.org
Cc: chenbo.xia@intel.com; zhihong.wang@intel.com; maxime.coquelin@redhat.com; stable@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in helper macro


On 8/29/20 2:22 PM, Andrew Rybchenko wrote:
> On 8/14/20 12:21 PM, Vipul Ashri wrote:
>> Inside Macro ASSIGN_UNLESS_EQUAL(var, val), assignment to var is 
>> always failing as assignment done using var_ having local scope only.
>> This leads to TX packets not going out and found broken due to 
>> cleanup malfunctioning. This patch fixes the wrong variable assignment.
>>
>> Fixes: 57f90f894588 ("net/virtio: reuse packed ring functions")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Vipul Ashri <vipul.ashri@oracle.com>
>> ---
>>   drivers/net/virtio/virtqueue.h | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/virtio/virtqueue.h 
>> b/drivers/net/virtio/virtqueue.h index 105a9c00c..20c95471e 100644
>> --- a/drivers/net/virtio/virtqueue.h
>> +++ b/drivers/net/virtio/virtqueue.h
>> @@ -607,10 +607,8 @@ virtqueue_notify(struct virtqueue *vq)
>>   
>>   /* avoid write operation when necessary, to lessen cache issues */
>>   #define ASSIGN_UNLESS_EQUAL(var, val) do {	\
>> -	typeof(var) var_ = (var);		\
>> -	typeof(val) val_ = (val);		\
>> -	if ((var_) != (val_))			\
>> -		(var_) = (val_);		\
>> +	if ((var) != (val))			\
>> +		(var) = (val);			\
> 
> Good catch. As I understand the old code tries to avoid processing of 
> var and val expressions twice. It looks it could be kept for val at 
> least. Just keep if condition as in old code and fix the last line 
> above:
>      (var) = val_;
> Since var_ and val_are local variables there is no necessity to 
> enclose it in parenthesis (but does not harm if done).
> var_ may be really removed since since resulting code will use it only 
> once.

I think there is a solution to avoid multiple evaluations of parameters:

// var is definitely an lvalue, so its address can definitely be taken #define ASSIGN_UNLESS_EQUAL(var, val) do {  \
	typeof(var) *const var_ = &(var);   \
	typeof(val)  const val_ = (val);    \
	if (*var_ != val_)                  \
		*var_ = val_;               \
} while (0)

The solution relies on the compiler optimizer. Here is the comparison of what the variants are compiled into:

https://urldefense.com/v3/__https://godbolt.org/z/nnvq5q__;!!GqivPVa7Brio!MJZCNAgcCHB5A_T1mI2aA-2F9wvqh_WOfzkeN0IbDnsZSlNIPWqDF0b4YDmTc19HcA$

  reply	other threads:[~2020-09-18 10:46 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200814092138.1692-1-vipul.ashri@oracle.com>
2020-08-29 11:22 ` Andrew Rybchenko
2020-08-29 22:17   ` Edward Makarov
2020-09-18 10:46     ` Vipul Ashri [this message]
2020-09-18 10:53       ` Maxime Coquelin
2020-09-18 10:41   ` Vipul Ashri

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