Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Andi Kleen (andi@firstfloor.org) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> writes:
> 
> > Both the signal-based and the sys_membarrier userspace RCU schemes
> > permit us to remove the memory barrier from the userspace RCU
> > rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
> > accelerating them. These memory barriers are replaced by compiler
> > barriers on the read-side, and all matching memory barriers on the
> > write-side are turned into an invokation of a memory barrier on all
> > active threads in the process. By letting the kernel perform this
> > synchronization rather than dumbly sending a signal to every process
> > threads (as we currently do), we diminish the number of unnecessary wake
> > ups and only issue the memory barriers on active threads. Non-running
> > threads do not need to execute such barrier anyway, because these are
> > implied by the scheduler context switches.
> 
> I'm not sure all this effort is really needed on architectures
> with strong memory ordering.

Do we still have many out there that support SMP ? Even newer ARM now
need memory barriers.

> 
> > + * The current implementation simply executes a memory barrier in an IPI handler
> > + * on each active cpu. Going through the hassle of taking run queue locks and
> > + * checking if the thread running on each online CPU belongs to the current
> > + * thread seems more heavyweight than the cost of the IPI itself.
> > + */
> > +SYSCALL_DEFINE0(membarrier)
> > +{
> > +	on_each_cpu(membarrier_ipi, NULL, 1);
> 
> Can't you use mm->cpu_vm_mask?

I'll go for PeterZ scheme, which is based on cpu_vm_mask.

Thanks,

Mathieu


> 
> -Andi

-- 
Mathieu Desnoyers
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