Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[John Hubbard]

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