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

[Joel Schopp]

> > Anyway, here is some disassembly of some of the code generated with my 
> > comments:
> >
> > c00000000049bf9c <.mutex_lock>:
> > c00000000049bf9c:       7c 00 06 ac     eieio
> > c00000000049bfa0:       7d 20 18 28     lwarx   r9,r0,r3
> > c00000000049bfa4:       31 29 ff ff     addic   r9,r9,-1
>
>
> > The eieio is completly unnecessary, it got picked up from 
> > atomic_dec_return (Anton, why is there an eieio at the start of 
> > atomic_dec_return in the first place?).
>
>
> a mutex is like a spinlock, it must prevent loads and stores within the 
> critical section from 'leaking outside the critical section' [they must 
> not be reordered to before the mutex_lock(), nor to after the 
> mutex_unlock()] - hence the barriers added by atomic_dec_return() are 
> very much needed.

The bne- and isync together form a sufficient import barrier.  See PowerPC Book2 
Appendix B.2.1.1

And if the eieio was necessary it should come after not before twidling the lock 
bits.
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