Messages in this thread | | | Date | Tue, 16 Jun 2015 17:33:24 -0700 | From | Alexei Starovoitov <> | Subject | Re: call_rcu from trace_preempt |
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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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