Re: [PATCH 2/3] net/ionic: remove duplicate barriers

["Boyer, Andrew" <Andrew.Boyer@amd.com>]

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On Feb 19, 2024, at 9:02 PM, Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com> wrote:

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-----Original Message-----
From: Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com<mailto:wathsala.vithanage@arm.com>>
Sent: Monday, February 19, 2024 4:17 PM
To: Andrew Boyer <andrew.boyer@amd.com<mailto:andrew.boyer@amd.com>>; dev@dpdk.org<mailto:dev@dpdk.org>
Cc: Neel Patel <neel.patel@amd.com<mailto:neel.patel@amd.com>>; nd <nd@arm.com<mailto:nd@arm.com>>; Honnappa
Nagarahalli <Honnappa.Nagarahalli@arm.com<mailto:Honnappa.Nagarahalli@arm.com>>; nd <nd@arm.com<mailto:nd@arm.com>>
Subject: RE: [PATCH 2/3] net/ionic: remove duplicate barriers

Hi Andrew,

I think that this barrier may have been added to ensure any writes to q-
hw_index and q->head_idx complete before ionic_q_flush computes val.
Dependency chains can also prevent reordering if that's the case this barrier is
not required.
However, I have the following concern.

diff --git a/drivers/net/ionic/ionic_main.c
b/drivers/net/ionic/ionic_main.c index 1f24f64a33..814bb3b8f4 100644
--- a/drivers/net/ionic/ionic_main.c
+++ b/drivers/net/ionic/ionic_main.c
@@ -223,7 +223,6 @@ ionic_adminq_post(struct ionic_lif *lif, struct
ionic_admin_ctx *ctx)
   q->head_idx = Q_NEXT_TO_POST(q, 1);

   /* Ring doorbell */
-   rte_wmb();
   ionic_q_flush(q);

err_out:

Ionic_q_flush(q) uses q->hw_index and q->head_idx to compute the value of
val which it writes to the address stored in q->db. I can see that q->head_idx is
updated before the removed rte_wmb(), therefore it's safe to assume that " q-
head_idx = Q_NEXT_TO_POST(q, 1)" and "val = IONIC_DBELL_QID(q-
hw_index) | q->head_idx" will maintain the program order due to that
dependency. But I don't know if there exists a dependency chain over q-
hw_index. If not, then that may have been the motive behind this barrier.


Since q->hw_index is also updated in the same CPU ionic_q_flush will
always see the correct value, consequently val should be always correct.
It's safe to remove this barrier.


Thank you for the careful review. We agree with this analysis.

-Andrew


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