linux-kernel - Re: call_rcu from trace

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Message-ID: <558209B8.80405@plumgrid.com>
Date:	Wed, 17 Jun 2015 16:58:48 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Daniel Wagner <daniel.wagner@...-carit.de>,
	LKML <linux-kernel@...r.kernel.org>, rostedt@...dmis.org
Subject: Re: call_rcu from trace_preempt

On 6/17/15 2:36 PM, Paul E. McKenney wrote:
> Well, you do need to have something in each element to allow them to be
> tracked.  You could indeed use llist_add() to maintain the per-CPU list,
> and then use llist_del_all() bulk-remove all the elements from the per-CPU
> list.  You can then pass each element in turn to kfree_rcu().  And yes,
> I am suggesting that you open-code this, as it is going to be easier to
> handle your special case then to provide a fully general solution.  For
> one thing, the general solution would require a full rcu_head to track
> offset and next.  In contrast, you can special-case the offset.  And
> ignore the overload special cases.

yes. all makes sense.

 > Locklessly enqueue onto a per-CPU list, but yes.  The freeing is up to

yes. per-cpu llist indeed.

 > you -- you get called just before exit from __call_rcu(), and get to
 > figure out what to do.
 >
 > My guess would be if not in interrupt and not recursively invoked,
 > atomically remove all the elements from the list, then pass each to
 > kfree_rcu(), and finally let things take their course from there.
 > The llist APIs look like they would work.

Above and 'just before the exit from __call_rcu()' part of suggestion
I still don't understand.
To avoid reentry into call_rcu I can either create 1 or N new kthreads
or work_queue and do manual wakeups, but that's very specialized and I
don't want to permanently waste them, so I'm thinking to llist_add into
per-cpu llists and do llist_del_all in rcu_process_callbacks() to take
them from these llists and call kfree_rcu on them.
The llist_add part will also do:
if (!rcu_is_watching()) invoke_rcu_core();
to raise softirq when necessary.
So at the end it will look like two phase kfree_rcu.
I'll try to code it up and see it explodes :)

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