Re: [PATCH linux-2.6 04/04] brsem: convert cpucontrol to brsem

[Nick Piggin <nickpiggin@yahoo.com.au>]

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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