Re: [RFC PATCH v13] sys_membarrier(): system/process-wide memory barrier (x86)

[Peter Zijlstra]

On Tue, Mar 17, 2015 at 10:57:50AM -0700, josh@xxxxxxxxxxxxxxxx wrote:
> On Tue, Mar 17, 2015 at 06:30:35PM +0100, Peter Zijlstra wrote:
> > On Tue, Mar 17, 2015 at 01:22:02PM -0400, Mathieu Desnoyers wrote:
> > > Here is an implementation of a new system call, sys_membarrier(), which
> > > executes a memory barrier on either all running threads of the current
> > > process (MEMBARRIER_PRIVATE) issues a memory barrier on all threads
> > > running on the system (~MEMBARRIER_PRIVATE). Both are currently
> > > implemented by calling synchronize_sched().
> > 
> > Then why bother with the flag?
> 
> Semantically, MEMBARRIER_PRIVATE is allowed to avoid issuing a barrier
> on CPUs not running the current process if it can, while
> ~MEMBARRIER_PRIVATE may not.  (The latter would be useful for
> applications such as system-wide tracing.)  That they're currently both
> implemented the same way doesn't mean they're semantically equivalent.

Sure; but why bother with pointless fluff like that? We can always
introduce the private flag if and when it starts to make sense having
it.
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