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Message-ID: <778f60b7-97c7-3c35-1b40-40cf844fe89e@nvidia.com>
Date: Tue, 6 Sep 2022 16:16:42 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>,
Jason Gunthorpe <jgg@...dia.com>
Cc: Yang Shi <shy828301@...il.com>, peterx@...hat.com,
kirill.shutemov@...ux.intel.com, hughd@...gle.com,
akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse
On 9/6/22 07:44, David Hildenbrand wrote:
>> Though it is all very unlikely, the general memory model standard is
>> to annotate with READ_ONCE.
>
> The only thing I could see going wrong in the comparison once the stars
> alingn would be something like the following:
>
> if (*a != b)
>
> implemented as
>
> if ((*a).lower != b.lower && (*a).higher != b.higher)
>
>
> This could only go wrong if we have more than one change such that:
>
> Original:
>
> *a = 0x00000000ffffffffull;
>
>
> First modification:
> *a = 0xffffffffffffffffull;
>
> Second modification:
> *a = 0x00000000eeeeeeeeull;
>
>
> If we race with both modifications, we could see that ffffffff matches,
> and could see that 00000000 matches as well.
>
>
> So I agree that we should change it, but not necessarily as an urgent
> fix and not necessarily in this patch. It's best to adjust all gup_*
> functions in one patch.
>
We had a long thread with Paul McKenney back in May [1] about this exact
sort of problem.
In that thread, I recall that "someone" tried to claim that a bare
one-byte read was safe, and even that innocent-sounding claim got
basically torn apart! :) Because the kernel memory model simply does
not cover you for bare reads and writes to shared mutable memory.
Unfortunately, until now, I'd only really remembered the conclusion:
"use READ_ONCE() and WRITE_ONCE() for any touching of shared mutable
memory", and not the point about other memory barriers not covering this
aspect. Thanks to Jason for reminding us of this. This time I think I
will remember it well enough to avoid another long thread. Maybe.
[1] https://lore.kernel.org/all/20220524163728.GO1790663@paulmck-ThinkPad-P17-Gen-1/
thanks,
--
John Hubbard
NVIDIA
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