linux-kernel - Re: call_rcu from trace

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Date:	Tue, 16 Jun 2015 17:33:24 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Daniel Wagner <wagi@...om.org>, paulmck@...ux.vnet.ibm.com,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: call_rcu from trace_preempt

On 6/16/15 10:37 AM, Steven Rostedt wrote:
>>> +			kfree(l);
>> >
>> >that's not right, since such thread defeats rcu protection of lookup.
>> >We need either kfree_rcu/call_rcu or synchronize_rcu.
>> >Obviously the former is preferred that's why I'm still digging into it.
>> >Probably a thread that does kfree_rcu would be ok, but we shouldn't
>> >be doing it unconditionally. For all networking programs and 99%
>> >of tracing programs the existing code is fine and I don't want to
>> >slow it down to tackle the corner case.
>> >Extra spin_lock just to add it to the list is also quite costly.
> Use a irq_work() handler to do the kfree_rcu(), and use llist (lockless
> list) to add items to the list.

have been studying irq_work and llist... it will work, but it's quite
costly too. Every kfree_rcu will be replaced with irq_work_queue(),
which is irq_work_claim() with one lock_cmpxchg plus another
lock_cmpxchg in llist_add, plus another lock_cmpxchg for our own llist
of 'to be kfree_rcu-ed htab elements'. That's a lot.
The must be better solution. Need to explore more.

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