Re: [RFC patch] introduce sys_membarrier(): process-wide memory barrier (v9)

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 2010-02-25 at 11:53 -0500, Mathieu Desnoyers wrote:

> > It would be very trivial compared to the process-private case. Just IPI
> > all CPUs. It would allow older kernels to work with newer process based
> > apps as they get implemented. But... not a really big deal I suppose.
> 
> This is actually what I did in v1 of the patch, but this implementation met
> resistance from the RT people, who were concerned about the impact on RT tasks
> of a lower priority process doing lots of sys_membarrier() calls. So if we want
> to do other-process-aware sys_membarrier(), we would have to iterate on all
> cpus, for every running process shared memory maps and see if there is something
> shared with all shm of the current process. This is clearly not as trivial as
> just broadcasting the IPI to all cpus.

Right, it may require another syscall or parameter to let the tasks
register a shared page. Then have some mechanism to find a way to
quickly check if a CPU is running a process with that page.

-- Steve