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

[Fenghua Yu]

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@xxxxxxxxx>
> > > > Tested-by: Tony Zhu <tony.zhu@xxxxxxxxx>
> > 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