linux-kernel - Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED

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Date:   Tue, 11 Aug 2020 09:54:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Chris Kennelly <ckennelly@...gle.com>,
        Peter Oskolkov <posk@...k.io>
Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add
 MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

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.