* Ingo Molnar ([EMAIL PROTECTED]) wrote: > > * Pekka Enberg <[EMAIL PROTECTED]> wrote: > > > Mathieu, Christoph is on vacation and I'm not at all that familiar > > with this cmpxchg_local() optimization, so if you could take a peek at > > this bug report to see if you can spot something obviously wrong with > > it, I would much appreciate that. > > hm, it's bad for at least one other reason as well (which is probably > unrelated to this crash): > > /* > * Currently fastpath is not supported if preemption is enabled. > */ > #if defined(CONFIG_FAST_CMPXCHG_LOCAL) && !defined(CONFIG_PREEMPT) > #define SLUB_FASTPATH > #endif > > such !PREEMPT exceptions tend to show "i didnt want to think too hard > about the preemptible case so just turn it off" thinking. >
Ingo, a comment in slub.c explains it : /* * The SLUB_FASTPATH path is provisional and is currently disabled if the * kernel is compiled with preemption or if the arch does not support * fast cmpxchg operations. There are a couple of coming changes that will * simplify matters and allow preemption. Ultimately we may end up making * SLUB_FASTPATH the default. * * 1. The introduction of the per cpu allocator will avoid array lookups * through get_cpu_slab(). A special register can be used instead. * * 2. The introduction of per cpu atomic operations (cpu_ops) means that * we can realize the logic here entirely with per cpu atomics. The * per cpu atomic ops will take care of the preemption issues. */ So there is more coming in the preemption area. > Also, why isnt this "SLUB_FASTPATH" flag done in the Kconfig space? > Eventually, I think only CONFIG_FAST_CMPXCHG_LOCAL will be needed (when the code will support preemption). Therefore, this SLUB_FASTPATH define seems to be only here temporarily. I'm looking at the code right now.. more to come. Mathieu > Ingo -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
