Re: [PATCH v8 4/6] MCS Lock: Move mcs_lock/unlock function into its own

[Tim Chen <tim.c.chen@linux.intel.com>]

On Tue, 2014-01-21 at 11:41 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Mon, Jan 20, 2014 at 05:24:31PM -0800, Tim Chen wrote:
> > > +EXPORT_SYMBOL_GPL(mcs_spin_lock);
> > > +EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> > 
> > Do we really need the EXPORTs? The only user so far is mutex and that's
> > core code. The other planned users are rwsems and rwlocks, for both it
> > would be in the slow path, which is also core code.
> >
> > We should generally only add EXPORTs once theres a need.
> 
> In fact I'd argue the hot path needs to be inlined.
> 
> We only don't inline regular locking primitives because it would blow 
> up the kernel's size in too many critical places.
> 
> But inlining an _internal_ locking implementation used in just a 
> handful of places is a no-brainer...
> 

The original mspin_lock primitive from which mcs_spin_lock was derived
has an explicit noinline annotation.  The comment says that it is so 
that perf can properly account for time spent in the lock function.  So
it wasn't inlined in previous kernels when we started.

For the time being, I'll just remove the EXPORT.  If people feel that
inline is the right way to go, then we'll leave the function
in mcs_spin_lock.h and not create mcs_spin_lock.c.


Tim


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