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Re: TSX lock elision for glibc v12


On Fri, 2013-06-21 at 12:48 +0200, Torvald Riegel wrote:
> On Thu, 2013-06-20 at 21:23 -0400, Rich Felker wrote:
> > On Thu, Jun 20, 2013 at 08:34:31PM +0200, Torvald Riegel wrote:
> > > Carlos asked me to send a more detailed plan for how we could split up
> > > Andi's work so that we can commit the parts that we can (hopefully) get
> > > consensus on before the freeze.  This might have some rough edges, but
> > > should show what I have in mind.  Comments?
> > > 
> > > 1) Give PTHREAD_MUTEX_DEFAULT a new enum value != 0 (and thus not equal
> > > to PTHREAD_MUTEX_NORMAL's value)
> > 
> > I agree this would be a better choice than changing the value of
> > PTHREAD_MUTEX_NORMAL. However, I have one additional idea: what about
> > swapping the two values when they move from pthread_mutexattr_t to
> > pthread_mutex_t? That way, existing mutexes initialized by
> > PTHREAD_MUTEX_INITIALIZER would have default type (and thus could use
> > elision) rather than having normal type. This is the approach we'll
> > probably use in musl if we add elision at some point.
> 
> Right, and that's the direction we discussed in the 2.18 thread just a
> while before you posted this.  Please have a look.

Attached is a start of a patch to clarify what I mean.  What is
obviously missing is handling the new PTHREAD_MUTEX_NORMAL_INT_NP in all
of the mutex functions, but that should be rather straight-forward, I
assume.

Comments?

> > On the other hand, glibc has non-default-type mutex initializer
> > macros, so we might have to consider whether any of them are
> > documented to have the "normal" behavior rather than the "default"
> > behavior. I'm not clear on this.

I think the other initializers are fine because they initialize to
kinds != DEFAULT already, so there's no ambiguity there, and we can just
decide whether we want to use elision for those or not.


Attachment: patch
Description: Text document


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