Messages in this thread | | | Date | Thu, 05 Jan 2006 10:42:14 -0600 | From | Joel Schopp <> | Subject | Re: [patch 00/21] mutex subsystem, -V14 |
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>>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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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