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

From: Mathieu Desnoyers
Date: Tue Mar 17 2015 - 15:01:36 EST

----- Original Message -----
> 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.

Without the expedited implementation, the only usefulness of the
private flag is to skip synchronize_sched() if called from a
single-threaded process.

We could easily argue that if a process is using sys_membarrier in
the first place, it's very likely that it is multithreaded. So I
agree that we can drop the flag for now, and add it later on,
e.g. when adding the expedited mode.

I am tempted to leave the "flags" argument in place though, along
with the "MEMBARRIER_QUERY" flag. Thoughts ?



Mathieu Desnoyers
EfficiOS Inc.
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