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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <1b510ced-ba3a-4bf9-8d33-204073cfedba@default> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=maxime.coquelin@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in helper macro X-BeenThere: stable@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches for DPDK stable branches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: stable-bounces@dpdk.org Sender: "stable" On 9/18/20 12:46 PM, Vipul Ashri wrote: > 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! Thanks Vipul, I plan to pick your patch later today. So I will assume Andrew still ack the patch since v5 is only fixing build. Thanks, Maxime > Regards > Vipul > > -----Original Message----- > From: Edward Makarov [mailto:makarov@kraftway.ru] > Sent: Sunday, 30 August, 2020 3:48 > To: Andrew Rybchenko ; Vipul Ashri ; 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 >>> --- >>> 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$ >