Re: [PATCH for 4.16 04/10] membarrier: provide SHARED_EXPEDITED command (v2)

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

----- On Jan 16, 2018, at 1:20 PM, Thomas Gleixner tglx@linutronix.de wrote:

> On Mon, 15 Jan 2018, Mathieu Desnoyers wrote:
>> +static int membarrier_shared_expedited(void)
>> +{
>> +	int cpu;
>> +	bool fallback = false;
>> +	cpumask_var_t tmpmask;
>> +
>> +	if (num_online_cpus() == 1)
>> +		return 0;
>> +
>> +	/*
>> +	 * Matches memory barriers around rq->curr modification in
>> +	 * scheduler.
>> +	 */
>> +	smp_mb();	/* system call entry is not a mb. */
>> +
>> +	/*
>> +	 * Expedited membarrier commands guarantee that they won't
>> +	 * block, hence the GFP_NOWAIT allocation flag and fallback
>> +	 * implementation.
>> +	 */
>> +	if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
>> +		/* Fallback for OOM. */
>> +		fallback = true;
>> +	}
>> +
>> +	cpus_read_lock();
>> +	for_each_online_cpu(cpu) {
>> +		struct task_struct *p;
>> +
>> +		/*
>> +		 * Skipping the current CPU is OK even through we can be
>> +		 * migrated at any point. The current CPU, at the point
>> +		 * where we read raw_smp_processor_id(), is ensured to
>> +		 * be in program order with respect to the caller
>> +		 * thread. Therefore, we can skip this CPU from the
>> +		 * iteration.
>> +		 */
>> +		if (cpu == raw_smp_processor_id())
>> +			continue;
>> +		rcu_read_lock();
>> +		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
>> +		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
>> +				   MEMBARRIER_STATE_SHARED_EXPEDITED)) {
> 
> This does not make sense vs. the documentation:
> 
>> + * @MEMBARRIER_CMD_SHARED_EXPEDITED:
>> + *                          Execute a memory barrier on all running threads
>> + *                          part of a process which previously registered
>> + *                          with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.
> 
> This should say:
> 
>> + *                          Execute a memory barrier on all running threads
>> + *                          of all processes which previously registered
>> + *                          with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.

Good point, will fix.

> 
> And I really have to ask whether this should be named _GLOBAL_ instead of
> _SHARED_.
> 
> Hmm?

I agree with you that this behavior fits better a "global" definition
than a "shared" one, especially given that it does not target a specific
shared memory mapping. The main issue I have is due to the pre-existing
MEMBARRIER_CMD_SHARED introduced in Linux 4.3. That one should also have
been called "MEMBARRIER_CMD_GLOBAL" based on the current line of thoughts.

Do you envision a way to transition forward to a new "MEMBARRIER_CMD_GLOBAL" for
the currently existing MEMBARRIER_CMD_SHARED ?

Perhaps with a duplicated enum entry ?

enum membarrier_cmd {
        MEMBARRIER_CMD_QUERY                                    = 0,
        MEMBARRIER_CMD_SHARED                                   = (1 << 0), /* use MEMBARRIER_CMD_GLOBAL instead */
        MEMBARRIER_CMD_GLOBAL                                   = (1 << 0),
[...]
};

Thanks,

Mathieu

> 
> Thanks,
> 
> 	tglx

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