Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 11, 2020 at 08:27:33AM +0200, Peter Zijlstra wrote:
>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
>  {
> +	int cflags = 0, int cpuid = -1;
> +
>  	if (unlikely(flags) && cmd != MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
>  		return -EINVAL;
> +
> +	if (cmd & (MEMBARRIER_CMD_PRIVATE_EXPEDITED |
> +		   MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE |
> +		   MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)) {
> +
> +		if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
> +			cflags |= MEMBARRIER_FLAG_RSEQ;
> +
> +		if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE) {
> +			cflags |= MEMBARRIER_FLAG_SYNC_CORE;
> +			cpuid = flags;
> +		}
> +
> +		cmd = MEMBARRIER_CMD_PRIVATE_EXPEDITED;
> +	}

This of course fails to check if other bits are set, and it goes really
'funny' if you use cpuid != -1.

That all needs a little more thought.