Re: [patch V3 05/12] rseq: Add prctl() to enable time slice extensions

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-10-29 09:22, Thomas Gleixner wrote:
> Implement a prctl() so that tasks can enable the time slice extension
> mechanism. This fails, when time slice extensions are disabled at compile
> time or on the kernel command line and when no rseq pointer is registered
> in the kernel.

I'm still unsure that going for enabling per-thread vs per-process is
the right approach. Enabling per-thread requires to either modify each
thread's startup code, or integrate this into libc's thread startup.

Enabling per-process makes it easy to invoke from program or library
constructor.

[...]

>   
> +int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
> +{

[...]

> +	case PR_RSEQ_SLICE_EXTENSION_SET: {
> +		u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> +		bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE);
> +
> +		if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE)
> +			return -EINVAL;
> +		if (!rseq_slice_extension_enabled())
> +			return -ENOTSUPP;
> +		if (!current->rseq.usrptr)
> +			return -ENXIO;
> +

So what happens if we have an (unlikely) scenario of:

- thread startup
- thread registration to rseq
- prctl PR_RSEQ_SLICE_EXTENSION_SET
- rseq unregistration
- rseq registration
--> What's the status of slice extension here ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com