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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
From: comex <comexk@gmail.com>
In-Reply-To: <174272a1-e21f-4d85-94ab-f0457bd1c93b@rowland.harvard.edu>
Date: Sun, 24 Mar 2024 13:37:03 -0400
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
 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>,
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 =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
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 Will Deacon <will@kernel.org>,
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 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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 Thomas Gleixner <tglx@linutronix.de>,
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 "H. Peter Anvin" <hpa@zytor.com>,
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References: <20240322233838.868874-1-boqun.feng@gmail.com>
 <s2jeqq22n5ef5jknaps37mfdjvuqrns4w7i22qp2r7r4bzjqs2@my3eyxoa3pl3>
 <CAHk-=whY5A=S=bLwCFL=043DoR0TTgSDUmfPDx2rXhkk3KANPQ@mail.gmail.com>
 <C85BE4F4-5847-45B4-A973-76B184B35EDE@gmail.com>
 <174272a1-e21f-4d85-94ab-f0457bd1c93b@rowland.harvard.edu>
To: Alan Stern <stern@rowland.harvard.edu>


> On Mar 24, 2024, at 11:22=E2=80=AFAM, Alan Stern =
<stern@rowland.harvard.edu> wrote:
>=20
> I don't know if this is what you meant by "in a weak memory model, the=20=

> escape can =E2=80=98time travel'".  Regardless, it seems very clear =
that any=20
> compiler which swaps L1 and L2 in f() has a genuine bug.

Yes, that=E2=80=99s what I meant.  Clang thinks it=E2=80=99s valid to =
swap L1 and L2.  Though, for it to actually happen, they would have to =
be in a loop, since the problematic optimization is =E2=80=9Cloop-invarian=
t code motion".  Here=E2=80=99s a modified version of your f() that =
shows the optimization in action:

https://godbolt.org/z/bdaTjjvMs

Anyway, my point is just that using LKMM doesn=E2=80=99t save you from =
the bug.=