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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
From: comex <comexk@gmail.com>
In-Reply-To: <CAHk-=whY5A=S=bLwCFL=043DoR0TTgSDUmfPDx2rXhkk3KANPQ@mail.gmail.com>
Date: Sat, 23 Mar 2024 17:40:23 -0400
Cc: Kent Overstreet <kent.overstreet@linux.dev>,
 Boqun Feng <boqun.feng@gmail.com>,
 rust-for-linux <rust-for-linux@vger.kernel.org>,
 linux-kernel@vger.kernel.org,
 linux-arch@vger.kernel.org,
 llvm@lists.linux.dev,
 Miguel Ojeda <ojeda@kernel.org>,
 Alex Gaynor <alex.gaynor@gmail.com>,
 Wedson Almeida Filho <wedsonaf@gmail.com>,
 Gary Guo <gary@garyguo.net>,
 =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
 Benno Lossin <benno.lossin@proton.me>,
 Andreas Hindborg <a.hindborg@samsung.com>,
 Alice Ryhl <aliceryhl@google.com>,
 Alan Stern <stern@rowland.harvard.edu>,
 Andrea Parri <parri.andrea@gmail.com>,
 Will Deacon <will@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>,
 Nicholas Piggin <npiggin@gmail.com>,
 David Howells <dhowells@redhat.com>,
 Jade Alglave <j.alglave@ucl.ac.uk>,
 Luc Maranget <luc.maranget@inria.fr>,
 "Paul E. McKenney" <paulmck@kernel.org>,
 Akira Yokosawa <akiyks@gmail.com>,
 Daniel Lustig <dlustig@nvidia.com>,
 Joel Fernandes <joel@joelfernandes.org>,
 Nathan Chancellor <nathan@kernel.org>,
 Nick Desaulniers <ndesaulniers@google.com>,
 kent.overstreet@gmail.com,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 elver@google.com,
 Mark Rutland <mark.rutland@arm.com>,
 Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org,
 "H. Peter Anvin" <hpa@zytor.com>,
 Catalin Marinas <catalin.marinas@arm.com>,
 linux-arm-kernel@lists.infradead.org,
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References: <20240322233838.868874-1-boqun.feng@gmail.com>
 <s2jeqq22n5ef5jknaps37mfdjvuqrns4w7i22qp2r7r4bzjqs2@my3eyxoa3pl3>
 <CAHk-=whY5A=S=bLwCFL=043DoR0TTgSDUmfPDx2rXhkk3KANPQ@mail.gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>


> On Mar 22, 2024, at 8:12=E2=80=AFPM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:
>=20
> And when the compiler itself is fundamentally buggy, you're kind of
> screwed. When you roll your own, you can work around the bugs in
> compilers.

That may be true, but the LLVM issue you cited isn=E2=80=99t a good =
example.  In that issue, the function being miscompiled doesn=E2=80=99t =
actually use any barriers or atomics itself; only the scaffolding around =
it does.  The same issue would happen even if the scaffolding used LKMM =
atomics.

For anyone curious: The problematic optimization involves an allocation =
(=E2=80=98p=E2=80=99) that is initially private to the function, but is =
returned at the end of the function.   LLVM moves a non-atomic store to =
that allocation across an external function call (to =E2=80=98foo=E2=80=99=
).  This reordering would be blatantly invalid if any other code could =
observe the contents of the allocation, but is valid if the allocation =
is private to the function.  LLVM assumes the latter: after all, the =
pointer to it hasn=E2=80=99t escaped.  Yet.  Except that in a weak =
memory model, the escape can =E2=80=98time travel=E2=80=99...=