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

[josh]

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.

- Josh Triplett
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