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

[Andi Kleen <andi@firstfloor.org>]

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.

> + * 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?

-Andi