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Nick Piggin wrote: > Tejun Heo wrote: > >> +/* >> + * cpucontrol is a brsem used to synchronize cpu hotplug events. >> + * Invoking lock_cpu_hotplug() read-locks cpucontrol and no >> + * hotplugging events will occur until it's released. >> + * >> + * Unfortunately, brsem itself makes use of lock_cpu_hotplug() and >> + * performing brsem write-lock operations on cpucontrol deadlocks. >> + * This is avoided by... >> + * >> + * a. guaranteeing that cpu hotplug events won't occur during the >> + * write-lock operations, and >> + * >> + * b. skipping lock_cpu_hotplug() inside brsem. >> + * >> + * #a is achieved by acquiring and releasing cpucontrol_mutex outside >> + * cpucontrol write-lock. #b is achieved by skipping >> + * lock_cpu_hotplug() inside brsem if the current task is >> + * cpucontrol_mutex holder (is_cpu_hotplug_holder() test). >> + * >> + * Also, note that cpucontrol is first initialized with >> + * BRSEM_BYPASS_INITIALIZER and then initialized again with >> + * __create_brsem() instead of simply using create_brsem(). This is >> + * necessary as cpucontrol brsem gets used way before brsem subsystem >> + * becomes up and running. >> + * >> + * Until brsem is properly initialized, all brsem ops succeed >> + * unconditionally. cpucontrol becomes operational only after >> + * cpucontrol_init() is finished, which should be called after >> + * brsem_init_early(). >> + */ > > > Mmm, this is just insane IMO. > > Note that I happen to also think the idea (brsems) have merit, and > that cpucontrol may be one of the places where a sane implementation > would actually be useful... but at least when you're introducing > this kind of complexity anywhere, you *really* need to be able to > back it up with numbers. > > As far as the VFS race fix goes, I guess Al or someone else will > comment on its correctness. But I think it might be nicer to first > fix it with a regular rwsem and then show some numbers to justify > its conversion to a brsem. > > If you need interruptible rwsems, I almost got an implementation > working a while back, and David Howells recently said he was > interested in doing them... so that's not an impossibility. > > Nick > Hello, Nick. I do agree that it's absolutely ugly. I thought about ripping the 3-tage init'ing and cpu hotplug stuff as currently cpu hotplug locking isn't used frequently, but was just giving a shot. I'll strip this thing out. Thanks. -- tejun - 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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