linux-kernel - Re: [tip: locking/core] locking/atomic/x86: Introduce arch_try_cmpxchg64

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Message-ID: <F0416322-2D03-408E-BC77-E68F024DF3EF@alien8.de>
Date: Thu, 18 Apr 2024 12:54:00 +0200
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
CC: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
 Ingo Molnar <mingo@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Waiman Long <longman@...hat.com>, x86@...nel.org
Subject: Re: [tip: locking/core] locking/atomic/x86: Introduce arch_try_cmpxchg64_local()

On April 17, 2024 9:21:29 PM GMT+02:00, Uros Bizjak <ubizjak@...il.com> wrote:
>The above is perhaps a little unfortunate example taken from
>
>if (cmpxchg64(...))
>
>where the check is against zero. The compiler can optimize the check
>to a TEST insn in this particular case, but otherwise CMP will be
>emitted for different usages. Not a big difference, but a register has
>to be kept live across cmpxchg8b.
>
...
>
>There is one important issue. When a register (or two for double-word
>values) has to be kept live for a compare, the register pressure on
>32bit targets around cmpxchg8b goes through the roof, and when using
>the frame pointer (and maybe some fixed register, e.g. PIC), the
>register allocator runs out of available registers. The number of
>spills around cmpxchg8b signals the troubles register allocator goes
>through to "fix" everything, so from the compiler PoV any relief is
>more than welcome here. Even in GCC internal libraries, we have had to
>take a special approach with this insn to avoid internal compiler
>errors. The kernel was quite lucky here ;)

That would've been a lot better reason to justify the change. I think you should put those things in the commit messages.

Thx.


-- 
Sent from a small device: formatting sucks and brevity is inevitable.