[PATCH] futex: improve user space accesses

[Andreas Schwab]

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 at linux-m68k.org
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