Re: [patch 00/21] mutex subsystem, -V14

[Linus Torvalds]

On Wed, 4 Jan 2006, Nicolas Pitre wrote:

> On Wed, 4 Jan 2006, Joel Schopp wrote:
> 
> > > this is version 14 of the generic mutex subsystem, against v2.6.15.
> > > 
> > > The patch-queue consists of 21 patches, which can also be downloaded from:
> > > 
> > >   http://redhat.com/~mingo/generic-mutex-subsystem/
> > > 
> > 
> > Took a glance at this on ppc64.  Would it be useful if I contributed an arch
> > specific version like arm has?  We'll either need an arch specific version or
> > have the generic changed.
> 
> Don't change the generic version.  You should provide a ppc specific 
> version if the generic ones don't look so good.

Well, if the generic one generates _buggy_ code on ppc64, that means that 
either the generic version is buggy, or one of the atomics that it uses is 
buggily implemented on ppc64.

And I think it's the generic mutex stuff that is buggy. It seems to assume 
memory barriers that aren't valid to assume.

A mutex is more than just updating the mutex count properly. You also have 
to have the proper memory barriers there to make sure that the things that 
the mutex _protects_ actually stay inside the mutex.

So while a ppc64-optimized mutex is probably a good idea per se, I think 
the generic mutex code had better be fixed first and regardless of any 
optimized version.

On x86/x86-64, the locked instructions automatically imply the proper 
memory barriers, but that was just lucky, I think.

			Linus
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