Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 30 Mar 2023 15:16:21 +0000 | From | Benno Lossin <> | Subject | Re: [PATCH v3 12/13] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit` |
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On 30.03.23 16:58, David Laight wrote: > From: y86-dev@protonmail.com >> Sent: 29 March 2023 23:34 >> >> `UniqueArc::try_new_uninit` calls `Arc::try_new(MaybeUninit::uninit())`. >> This results in the uninitialized memory being placed on the stack, >> which may be arbitrarily large due to the generic `T` and thus could >> cause a stack overflow for large types. > > Does that mean rust is using (the equivalent of) alloca() ?
No, the compiler knows the size of `T` statically (this is a requirement for calling `MaybeUninit::uninit()`). It would simply reserve stack space for a stack variable (like the C compiler would also do) and since the type is generic and user-specified, it could be larger than the stack size. The problem is that the Rust compiler does not optimize the stack variable away. This fix avoids creating a stack variable in the first place.
-- Cheers, Benno
> > That is banned for C code in the kernel for any sizes. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales)
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