Messages in this thread | | | Date | Tue, 6 Sep 2022 16:44:20 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse |
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On 06.09.22 16:30, Jason Gunthorpe wrote: > On Tue, Sep 06, 2022 at 03:57:30PM +0200, David Hildenbrand wrote: > >>> READ_ONCE primarily is a marker that the data being read is unstable >>> and that the compiler must avoid all instability when reading it. eg >>> in this case the compiler could insanely double read the value, even >>> though the 'if' requires only a single read. This would result in >>> corrupt calculation. >> >> As we have a full memory barrier + compile barrier, the compiler might >> indeed do double reads and all that stuff. BUT, it has to re-read after we >> incremented the refcount, and IMHO that's the important part to detect the >> change. > > Yes, it is important, but it is not the only important part. > > The compiler still has to exectute "if (*a != b)" *correctly*. > > This is what READ_ONCE is for. It doesn't set order, it doesn't > implement a barrier, it tells the compiler that '*a' is unstable data > and the compiler cannot make assumptions based on the idea that > reading '*a' multiple times will always return the same value. > > If the compiler makes those assumptions then maybe even though 'if (*a > != b)' is the reality, it could mis-compute '*a == b'. You enter into > undefined behavior here. > > 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.
... I do wonder if we want to reuse ptep_get_lockless() instead of the READ_ONCE(). CONFIG_GUP_GET_PTE_LOW_HIGH is confusing.
-- Thanks,
David / dhildenb
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