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

[Joel Schopp <jschopp@austin.ibm.com>]

>>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.