Messages in this thread | | | Date | Tue, 26 Sep 2006 22:08:56 -0700 | Subject | Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1] | From | Bill Huey (hui) <> |
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On Tue, Sep 26, 2006 at 08:55:41PM -0600, Eric W. Biederman wrote: > Bill Huey (hui) <billh@gnuppy.monkey.org> writes: > > This patch moves put_task_struct() reaping into a thread instead of an > > RCU callback function as discussed with Esben publically and Ingo privately: > > Stupid question.
It's a great question actually.
> Why does the rt tree make all calls to put_task_struct an rcu action? > We only need the rcu callback from kernel/exit.c
Because the conversion of memory allocation routines like kmalloc and kfree aren't safely callable within a preempt_disable critical section since they were incompletely converted in the -rt. It can run into the sleeping in atomic scenario which can result in a deadlock since those routines use blocking locks internally in the implementation now as a result of the spinlock_t conversion to blocking locks.
> Nothing else needs those semantics.
Right, blame it on the incomplete conversion of the kmalloc and friends. GFP_ATOMIC is is kind of meaningless in the -rt tree and it might be a good thing to add something like GFP_RT_ATOMIC for cases like this to be handled properly and restore that particular semantic in a more meaningful way.
> I agree that put_task_struct is the most common point so this is unlikely > to remove your issues with rcu callbacks but it just seems completely backwards > to increase the number of rcu callbacks in the rt tree.
I'm not sure what mean here, but if you mean that you don't like the RCU API abuse then I agree with you on that. However, Ingo disagrees and I'm not going to argue it with him. Although, I'm not going stop you if you do. :)
bill
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