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Date:   Thu, 30 Mar 2023 15:16:21 +0000
To:     David Laight <David.Laight@ACULAB.COM>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Alice Ryhl <alice@ryhl.io>
From:   Benno Lossin <y86-dev@protonmail.com>
Cc:     "rust-for-linux@vger.kernel.org" <rust-for-linux@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "patches@lists.linux.dev" <patches@lists.linux.dev>
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.
>
> =09David
>
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