Re: [PATCH] introduce sys_membarrier(): process-wide memory barrier (v10)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Josh Triplett (josh@joshtriplett.org) wrote:
> On Mon, Apr 05, 2010 at 03:10:57PM -0400, Mathieu Desnoyers wrote:
> > * Randy Dunlap (randy.dunlap@oracle.com) wrote:
> > > On Mon, 5 Apr 2010 13:57:37 -0400 Mathieu Desnoyers wrote:
> > > > +/*
> > > > + * sys_membarrier - issue memory barrier on current process running threads
> > > > + * @flags: One of these must be set:
> > > > + *         MEMBARRIER_EXPEDITED
> > > > + *             Adds some overhead, fast execution (few microseconds)
> > > > + *         MEMBARRIER_DELAYED
> > > > + *             Low overhead, but slow execution (few milliseconds)
> > > > + *
> > > > + *         MEMBARRIER_QUERY
> > > > + *           This optional flag can be set to query if the kernel supports
> > > > + *           a set of flags.
> > > > + *
> > > > + * return values: Returns -EINVAL if the flags are incorrect. Testing for kernel
> > > > + * sys_membarrier support can be done by checking for -ENOSYS return value.
> > > > + * Return values >= 0 indicate success. For a given set of flags on a given
> > > > + * kernel, this system call will always return the same value. It is therefore
> > > > + * correct to check the return value only once at library load, passing the
> > > 
> > > library load assumes caller is a library?  does the kernel care about that?
> > 
> > Nope, it doesn't. Will rephrase:
> > 
> >     ... It is therefore
> >  * correct to check the return value only once during a process lifetime,
> >  * passing the MEMBARRIER_QUERY flag in addition to only check if the flags are
> >  * supported, without performing any synchronization.
> 
> Technically you can optimize even more than "process lifetime", since as
> you said the results hold "For a given set of flags on a given kernel".
> So you could check once and use the results as long as you remain on the
> same running system.  (Craziness like live process migration and
> checkpoint/restart aside. :) )

True. But notice that I first state the general correctness condition
(consistent on a given kernel), and then state that it is therefore true over a
process life-time. Basically, I want to clarify how the "MEMBARRIER_QUERY" flag
can be used from a user-space perspective. Stating that we can save the result
here and there and re-use it afterward will not help making this system call
documentation clearer; I fear it would just have the opposite effect: confusing
API users.

Thanks,

Mathieu

> 
> - Josh Triplett

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com