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Date:   Tue, 11 Aug 2020 09:54:20 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peter Oskolkov <posk@google.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>,
        Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add
 MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
Message-ID: <20200811075420.GA5637@worktop.programming.kicks-ass.net>
References: <20200811000959.2486636-1-posk@google.com>
 <20200811062733.GP3982@worktop.programming.kicks-ass.net>
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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.