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Message-ID: <c6c45aca-e8b9-f10e-181a-f0ca8a79a64c@intel.com>
Date: Tue, 30 Jan 2024 12:41:31 -0800
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Subject: Re: [PATCH] dmaengine: idxd: Change wmb() to smp_wmb() when copying
 completion record to user space
Content-Language: en-US
To: Dave Hansen <dave.hansen@intel.com>, Boqun Feng <boqun.feng@gmail.com>,
	Mark Rutland <mark.rutland@arm.com>
CC: Vinod Koul <vkoul@kernel.org>, Dave Jiang <dave.jiang@intel.com>,
	<dmaengine@vger.kernel.org>, linux-kernel <linux-kernel@vger.kernel.org>,
	Nikhil Rao <nikhil.rao@intel.com>, Tony Zhu <tony.zhu@intel.com>, "Mathieu
 Desnoyers" <mathieu.desnoyers@efficios.com>, Thomas Gleixner
	<tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov
	<bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, <x86@kernel.org>
References: <20240130025806.2027284-1-fenghua.yu@intel.com>
 <Zbk4wGNcB-g91Vr0@FVFF77S0Q05N> <ZblTystHpVkvjbkv@boqun-archlinux>
 <388be136-f91c-403a-99e1-7a10c5bf9691@intel.com>
From: Fenghua Yu <fenghua.yu@intel.com>
In-Reply-To: <388be136-f91c-403a-99e1-7a10c5bf9691@intel.com>
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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@intel.com>
>>>> Tested-by: Tony Zhu <tony.zhu@intel.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