linux-kernel - Re: [WIP 0/3] Memory model and atomic API in Rust

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Message-Id: <3088DF9A-6507-423B-8F0A-100B78DE1A26@gmail.com>
Date: Sun, 24 Mar 2024 13:37:03 -0400
From: comex <comexk@...il.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Kent Overstreet <kent.overstreet@...ux.dev>,
 Boqun Feng <boqun.feng@...il.com>,
 rust-for-linux <rust-for-linux@...r.kernel.org>,
 linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org,
 llvm@...ts.linux.dev,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Andrea Parri <parri.andrea@...il.com>,
 Will Deacon <will@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Nicholas Piggin <npiggin@...il.com>,
 David Howells <dhowells@...hat.com>,
 Jade Alglave <j.alglave@....ac.uk>,
 Luc Maranget <luc.maranget@...ia.fr>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Akira Yokosawa <akiyks@...il.com>,
 Daniel Lustig <dlustig@...dia.com>,
 Joel Fernandes <joel@...lfernandes.org>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>,
 kent.overstreet@...il.com,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Marco Elver <elver@...gle.com>,
 Mark Rutland <mark.rutland@....com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>,
 Catalin Marinas <catalin.marinas@....com>,
 linux-arm-kernel@...ts.infradead.org,
 linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

> On Mar 24, 2024, at 11:22 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> 
> I don't know if this is what you meant by "in a weak memory model, the 
> escape can ‘time travel'".  Regardless, it seems very clear that any 
> compiler which swaps L1 and L2 in f() has a genuine bug.

Yes, that’s what I meant.  Clang thinks it’s 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 “loop-invariant code motion".  Here’s 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’t save you from the bug.