linux-kernel - Re: [PATCH 1/2 v7] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED

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Message-ID: <1703212153.12161.1600200508335.JavaMail.zimbra@efficios.com>
Date:   Tue, 15 Sep 2020 16:08:28 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     paulmck <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <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 v7] rseq/membarrier: add
 MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

----- On Sep 15, 2020, at 2:55 PM, Peter Oskolkov posk@...gle.com wrote:
[...]
> 
> -static int membarrier_private_expedited(int flags)
> +static int membarrier_private_expedited(int flags, int cpu_id)
> {
> -	int cpu;
> 	cpumask_var_t tmpmask;
> 	struct mm_struct *mm = current->mm;
> +	smp_call_func_t ipi_func = ipi_mb;
> 
> -	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
> +	if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
> 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> 			return -EINVAL;
> 		if (!(atomic_read(&mm->membarrier_state) &
> 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
> 			return -EPERM;
> +	} else if (flags == MEMBARRIER_FLAG_RSEQ) {
> +		if (!IS_ENABLED(CONFIG_RSEQ))
> +			return -EINVAL;
> +		if (!(atomic_read(&mm->membarrier_state) &
> +		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY))
> +			return -EPERM;
> +		ipi_func = ipi_rseq;
> 	} else {
> +		BUG_ON(flags != 0);

Usually BUG_ON() is really for utterly unrecoverable situations, which is not the
case here. I am tempted to just use WARN_ON_ONCE(flags) instead to make dmesg
yell (once) if an unexpected flags parameter is received. This is not a user-space
input, so it should never trigger unless we change the code.

The rest looks good to me, thanks!

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

Mathieu


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