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 12:30:56 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Boqun Feng <boqun.feng@...il.com>, Mark Rutland <mark.rutland@....com>
Cc: Fenghua Yu <fenghua.yu@...el.com>, 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

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?