linux-kernel - Re: [PATCH] dmaengine: idxd: Change wmb() to smp_wmb() when copying completion record to user space

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Date: Tue, 30 Jan 2024 12:41:31 -0800
From: Fenghua Yu <fenghua.yu@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, Boqun Feng <boqun.feng@...il.com>,
	Mark Rutland <mark.rutland@....com>
CC: Vinod Koul <vkoul@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
	<dmaengine@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>,
	Nikhil Rao <nikhil.rao@...el.com>, Tony Zhu <tony.zhu@...el.com>, "Mathieu
 Desnoyers" <mathieu.desnoyers@...icios.com>, Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
	<bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>
Subject: Re: [PATCH] dmaengine: idxd: Change wmb() to smp_wmb() when copying
 completion record to user space

Hi, Dave, Boqun, and Mark,

On 1/30/24 12:30, Dave Hansen wrote:
> On 1/30/24 11:53, Boqun Feng wrote:
>>>> Fixes: b022f59725f0 ("dmaengine: idxd: add idxd_copy_cr() to copy user completion record during page fault handling")
>>>> Suggested-by: Nikhil Rao <nikhil.rao@...el.com>
>>>> Tested-by: Tony Zhu <tony.zhu@...el.com>
>> Since it has a "Fixes" tag and a "Tested-by" tag, I'd assume there has
>> been a test w/ and w/o this patch showing it can resolve a real issue
>> *constantly*? If so, I think x86 might be broken somewhere.
>>
>> [Cc x86 maintainers]
> 
> Fenghua, could you perhaps explain how this problem affects end users?
> What symptom was observed that made it obvious something was broken and
> what changes with this patch?

There is no issue found by any test. This wmb() code was reviewed and 
was "thought" that it may have a potential issue. The patch was tested 
without breaking any existing tests.

 From the discussions with Boqun and Mark, this patch might just be an 
optimization rather than a fix. Let me think about it further and may 
update commit message in v2 or withdraw this patch since it won't really 
fix an issue.

Thank you very much for review!

-Fenghua