Messages in this thread | | | Date | Sun, 25 Sep 2005 17:39:25 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH linux-2.6 04/04] brsem: convert cpucontrol to brsem |
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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
-- SUSE Labs, Novell Inc.
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