[PATCH] nvme: use lighter smp barriers in nvme_irq

[Chaitanya Kulkarni]

Keith,

On 2/17/21 19:28, Keith Busch wrote:
> On Wed, Feb 17, 2021 at 08:26:30AM +0100, Heiner Kallweit wrote:
>> On 17.02.2021 00:59, Chaitanya Kulkarni wrote:
>>> On 2/14/21 08:30, Heiner Kallweit wrote:
>>>> Based on the description we should be fine using the less heavy-weight
>>>> smp barriers here. On x86 this would be compiler barriers only.
>>>>
>>>> Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
>>> Can you share performance numbers ?
>>>
>> I stumbled across this code piece by chance and don't have hw to test it.
>> Having said that the change is based on code inspection only.
>> Unfortunately the barrier comment is very generic and doesn't mention
>> a problematic scenario. Also the commit message of 3a7afd8ee42a doesn't
>> mention a potential race. Most likely the barrier can be removed
>> completely. Helpful would be if someone could explain the potential race
>> in detail, means which reordering / variable accesses could be racy.
>> Then we would have a basis to talk about READ_ONCE/WRITE_ONCE vs.
>> smp barrier vs. dma barrier vs. full barrier.
> The variables the driver was protecting no longer exist, so I also agree
> the barriers should not be necessary.
>

Are you saying something like following is needed ?

>From a3a73bd3943b479f82ff00582baf2c37c1afd8e8 Mon Sep 17 00:00:00 2001
From: Chaitanya Kulkarni <chaitanya.kulkarni at wdc.com>
Date: Wed, 17 Feb 2021 20:01:37 -0800
Subject: [PATCH] nvme-pci: remove the barriers

The variable which was protected by the barriers is removed in
The commit f6c4d97b0d82 (" nvme/pci: Remove last_cq_head").

Remove the barriers which was protecting the variable.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni at wdc.com>
---
 drivers/nvme/host/pci.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 0045c5edf629..3729775f6a8a 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -1062,14 +1062,8 @@ static irqreturn_t nvme_irq(int irq, void *data)
     struct nvme_queue *nvmeq = data;
     irqreturn_t ret = IRQ_NONE;
 
-    /*
-     * The rmb/wmb pair ensures we see all updates from a previous run of
-     * the irq handler, even if that was on another CPU.
-     */
-    rmb();
     if (nvme_process_cq(nvmeq))
         ret = IRQ_HANDLED;
-    wmb();
 
     return ret;
 }
-- 
2.22.1