Re: [PATCH v1 05/14] futex: Add sys_futex_wake()

On Fri, Jul 21, 2023, at 20:54, Peter Zijlstra wrote:

On Fri, Jul 21, 2023 at 05:41:20PM +0200, Arnd Bergmann wrote:

quoted

On Fri, Jul 21, 2023, at 12:22, Peter Zijlstra wrote:

quoted

--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -87,6 +87,7 @@ COND_SYSCALL_COMPAT(set_robust_list);
 COND_SYSCALL(get_robust_list);
 COND_SYSCALL_COMPAT(get_robust_list);
 COND_SYSCALL(futex_waitv);
+COND_SYSCALL(futex_wake);
 COND_SYSCALL(kexec_load);
 COND_SYSCALL_COMPAT(kexec_load);
 COND_SYSCALL(init_module);

This is fine for the moment, but I wonder if we should start making
futex mandatory at some point. Right now, sparc32 with CONFIG_SMP
cannot support futex because of the lack of atomics in early
sparc processors, but sparc32 glibc actually requires futexes
and consequently only works on uniprocessor machines, on sparc64
compat mode, or on Leon3 with out of tree patches.

PARISC is another 'fun' case.

I had to look up how that works, but as far as I can tell, the
parisc code actually has a chance of working, as the userspace
atomics go through the light-weight syscall that shares a hashed
lock with the actual futex syscall. On sparc32 I think it's
worse because userspace assumes that atomic instructions are
supported while the kernel assumes they are not.

        Arnd