linux-kernel - Re: [PATCH] futex: improve user space accesses

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Message-ID: <87bjxl6b0i.fsf@igel.home>
Date: Sun, 08 Dec 2024 23:54:53 +0100
From: Andreas Schwab <schwab@...ux-m68k.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>,  LKML
 <linux-kernel@...r.kernel.org>,  x86@...nel.org,
 linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] futex: improve user space accesses

On Nov 22 2024, Linus Torvalds wrote:

> Josh Poimboeuf reports that he got a "will-it-scale.per_process_ops 1.9%
> improvement" report for his patch that changed __get_user() to use
> pointer masking instead of the explicit speculation barrier.  However,
> that patch doesn't actually work in the general case, because some (very
> bad) architecture-specific code actually depends on __get_user() also
> working on kernel addresses.
>
> A profile showed that the offending __get_user() was the futex code,
> which really should be fixed up to not use that horrid legacy case.
> Rewrite futex_get_value_locked() to use the modern user acccess helpers,
> and inline it so that the compiler not only avoids the function call for
> a few instructions, but can do CSE on the address masking.

This breaks userspace on ppc32.  As soon as /init in the initrd is
started the kernel hangs (without any messages).

-- 
Andreas Schwab, schwab@...ux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."