Re: [PATCH 1/2 v7] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

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

----- On Sep 15, 2020, at 2:55 PM, Peter Oskolkov posk@google.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@efficios.com>

Mathieu


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