Re: [PATCH 4/4] sparc32: add the sparc_atomic_cmpxchg_32 system call

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Oct 15, 2026, at 10:55, Ramin Moussavi wrote:
> User space built for v8 has no compare-and-swap. C libraries emulate it
> with ldstub locks of their own, and those have two problems the kernel
> cannot see. They are not async-signal-safe: a handler that reaches an
> atomic while the code it interrupted holds the lock spins forever. And
> the kernel does not know them: FUTEX_WAKE_OP and robust futexes write
> the same words without taking those locks.

As far as I know, the baseline in sparc32 glibc has been leon3 or
sparcv9 for a long time, so CASA should always be available.

Any remaining sun4m users are on an ancient userspace with a
pre-futex glibc.

> Let user space ask the kernel instead. The call goes through
> futex_atomic_cmpxchg_inatomic(), so it agrees with whatever the futex
> code does to the same word, casa on LEON or the hashed lock otherwise,
> and a single system call cannot be interrupted by a signal halfway.
> m68k solved the same problem the same way with atomic_cmpxchg_32.

This sounds like actively regressing performance on LEON compared
to just using CASA and futex. What are you actually trying to
achieve?

> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl 
> b/arch/sparc/kernel/syscalls/syscall.tbl
> index 5b9fe0e8140f..939e0130b7e2 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -518,3 +518,4 @@
>  470	common	listns				sys_listns
>  471	common	rseq_slice_yield		sys_rseq_slice_yield
>  472	common	fchroot				sys_fchroot
> +473	32	sparc_atomic_cmpxchg_32		sys_sparc_atomic_cmpxchg_32	sys_ni_syscall

Wrong syscall number.

     Arnd