From: Sinan Kaya <okaya@codeaurora.org>
To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Timur Tabi <timur@codeaurora.org>,
Netdev <netdev@vger.kernel.org>,
sulrich@codeaurora.org, linux-arm-msm@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Jeff Kirsher <jeffrey.t.kirsher@intel.com>,
intel-wired-lan <intel-wired-lan@lists.osuosl.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 7/7] ixgbevf: eliminate duplicate barriers on weakly-ordered archs
Date: Thu, 15 Mar 2018 12:27:46 -0400 [thread overview]
Message-ID: <0175e460-3424-9838-1064-9f63dab3304f@codeaurora.org> (raw)
In-Reply-To: <eee8269d-b711-828c-ab84-5933bf86d024@codeaurora.org>
On 3/15/2018 12:21 PM, Sinan Kaya wrote:
> On 3/15/2018 10:32 AM, Alexander Duyck wrote:
>> We tend to do something like:
>> update tx_buffer_info
>> update tx_desc
>> wmb()
>> point first tx_buffer_info next_to_watch value at last tx_desc
>> update next_to_use
>> notify device via writel
>>
>> We do it this way because we have to synchronize between the Tx
>> cleanup path and the hardware so we basically lump the two barriers
>> together. instead of invoking both a smp_wmb and a wmb. Now that I
>> look at the pseudocode though I wonder if we shouldn't move the
>> next_to_use update before the wmb, but that might be material for
>> another patch. Anyway, in the Tx cleanup path we should have an
>> smp_rmb() after we read the next_to_watch values so that we avoid
>> reading any of the other fields in the buffer_info if either the field
>> is NULL or the descriptor pointed to has not been written back.
>
> How do you feel about keeping wmb() very close to writel_relaxed() like this?
>
> update tx_buffer_info
> update tx_desc
> point first tx_buffer_info next_to_watch value at last tx_desc
> update next_to_use
> wmb()
> notify device via writel_relaxed()
>
> I'm afraid that if the order of wmb() and writel() is not very
> obvious or hidden in multiple functions, somebody can introduce a very nasty
> bug in the future.
>
> We also have to think about code maintenance.
>
Now that I read your email again, I think this is the reason if I understood you
correctly.
"instead of invoking both a smp_wmb and a wmb"
You'd need something like
update tx_buffer_info
update tx_desc
smp_wmb()
point first tx_buffer_info next_to_watch value at last tx_desc
update next_to_use
wmb()
notify device via writel_relaxed()
Let me work on your comments.
--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
next prev parent reply other threads:[~2018-03-15 16:27 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-14 3:20 [PATCH 1/7] i40e/i40evf: Eliminate duplicate barriers on weakly-ordered archs Sinan Kaya
2018-03-14 3:20 ` [PATCH 2/7] ixgbe: eliminate " Sinan Kaya
2018-03-15 1:47 ` Alexander Duyck
2018-03-14 3:20 ` [PATCH 3/7] RDMA/qedr: " Sinan Kaya
2018-03-14 4:12 ` Jason Gunthorpe
2018-03-14 12:06 ` okaya
2018-03-15 22:23 ` Jason Gunthorpe
2018-03-14 3:20 ` [PATCH 4/7] igbvf: " Sinan Kaya
2018-03-15 1:48 ` Alexander Duyck
2018-03-14 3:20 ` [PATCH 5/7] igb: " Sinan Kaya
2018-03-15 1:50 ` Alexander Duyck
2018-03-14 3:20 ` [PATCH 6/7] e1000: " Sinan Kaya
2018-03-15 1:41 ` Alexander Duyck
2018-03-15 23:30 ` Sinan Kaya
2018-03-16 0:25 ` Alexander Duyck
2018-03-16 0:50 ` Sinan Kaya
2018-03-14 3:20 ` [PATCH 7/7] ixgbevf: " Sinan Kaya
2018-03-14 5:08 ` Timur Tabi
2018-03-14 12:13 ` okaya
2018-03-14 21:49 ` Alexander Duyck
2018-03-14 22:57 ` Sinan Kaya
2018-03-15 1:44 ` Alexander Duyck
2018-03-15 2:17 ` Sinan Kaya
2018-03-15 14:32 ` Alexander Duyck
2018-03-15 16:21 ` Sinan Kaya
2018-03-15 16:27 ` Sinan Kaya [this message]
2018-03-15 16:58 ` Alexander Duyck
2018-03-15 1:45 ` Alexander Duyck
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