Messages in this thread | | | Date | Mon, 13 Dec 2021 19:18:54 +0100 | From | Marco Elver <> | Subject | Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme |
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On Mon, Dec 13, 2021 at 10:11AM -0800, Linus Torvalds wrote: > On Mon, Dec 13, 2021 at 8:43 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > So Marco was expressing doubt about this exact interface for the > > atomic_*_overflow() functions, since it's extremely easy to get the > > whole ATOMIC_OVERFLOW_OFFSET thing wrong. > > I missed that discussion (maybe it was on irc? Or maybe I just get too > much email). > > Anyway, my preferred solution would simply be to make the ref-counting > atomics use a different type. > > Voilà, problem solved. You can't really misuse them by mistake, > because you can't access it by mistake. > > Sure, it could be a wrapper around 'atomic_t' on architectures that > end up using the generic fallback, so it might be as simple as > > typedef atomic_t atomic_ref_t; > > in some asm-generic implementation, although I suspect that you'd want > type safety even there, and do > > typedef struct { atomic_t atomic_val; } atomic_ref_t; > > But then on x86 - and other architectures that might prefer to use > that offset trick because they have flags - I'm not sure it even makes > sense to have anything to do with 'atomic_t' at all, since there would > basically be zero overlap with the regular atomic operations (partly > due to the offset, but partly simply because the 'ref' operations are > simply different). > > (Wrt naming: I do think this is more about the "ref" part than the > "overflow" part - thus I'd suggest the "atomic_ref_t" rather than your > ofl naming).
I'm still genuinely worried about this:
> 2. Yet another potentially larger issue is if some code > kmalloc()s some structs containing refcount_t, and relies on > GFP_ZERO (kzalloc()) to initialize their data assuming that a > freshly initialized refcount_t contains 0.
Even with everything properly wrapped up in atomic_ref_t, it's not going to prevent mis-initialization via kzalloc() and friends.
I think C won't let us design that misuse out of existence.
Thanks, -- Marco
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