Messages in this thread | | | Date | Mon, 28 Apr 2008 14:56:49 +0400 | From | Oleg Nesterov <> | Subject | Re: get_online_cpus() && workqueues |
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On 04/28, Gautham R Shenoy wrote: > > On Sat, Apr 26, 2008 at 06:43:30PM +0400, Oleg Nesterov wrote: > > > > Can't we add another nested lock which is dropped right after __cpu_die()? > > (in fact I think it could be dropped after __stop_machine_run). > > > > The new read-lock is get_online_map() (just a random name for now). The only > > difference wrt get_online_cpus() is that it doesn't protect against CPU_DEAD, > > but most users of get_online_cpus() doesn't need this, they only need a > > stable cpu_online_map and sometimes they need to be sure that some per-cpu > > object (say, cpu_workqueue_struct->thread) can't be destroyed under this > > lock. > > > > get_online_map() seem to fit for this, and can be used from work->func(). > > (actually, I think most users of use get_online_cpus() could use the new > > helper instead, but this doen't matter). > > However, subsystems such as cpufreq require serialization with respect > to the whole CPU-Hotplug operation since they do initialization and > cleanup pre and post the change of the cpu_online_map. > The current code, or this patch doesn't help in such cases > when such subsystems have multithreaded workqueues!
Yes, I see, thanks. Heiko has pointed this too.
> One of the thoughts I have is to provide an API along the lines of > try_get_online_cpus() which will return 1 if there is no CPU Hotplug > operation in progress and will return 0 otherwise. In case where > a cpu-hotplug operation is in progress, the workitem could simply > do nothing other than requeue itself and wait for the cpu-hotplug > operation to complete.
Yes, possible, but it is not nice that work->func() can't just use get_online_cpus()...
> Else, we might want to do something like what slab.c does. > It sets the per-cpu work.func of the cpu-going down to NULL in > CPU_DOWN_PREPARE.
Yes, but this is different. Please note also that this particular work must not use get_online_cpus(), no matter what changes we can make. Otherwise cancel_delayed_work_sync() can deadlock.
What do you think about another patch I sent? I am not happy with it, and it certainly uglifies cpu.c, but it is simple...
Oleg.
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