Re: call_rcu from trace_preempt

From: Alexei Starovoitov
Date: Wed Jun 17 2015 - 14:39:40 EST

On 6/17/15 2:05 AM, Daniel Wagner wrote:
>Steven's suggestion deferring the work via irq_work results in the same
>stack trace. (Now I get cold feets, without the nice heat from the CPU
>busy looping...)
That one still not working. It also makes the system really really slow.
I guess I still do something completely wrong.

tried your irq_work patch. It indeed makes the whole system
unresponsive. Ctrl-C of hwlathist no longer works and
it runs out of memory in 20 sec or so of running hwlathist
on idle system (without parallel hackbench).
It looks that free_pending flag is racy, so I removed it,
but it didn't help.

Also I've tried all sort of other things in rcu including
add rcu_bpf similar to rcu_sched to make sure that recursive
call into call_rcu will not be messing rcu_preempt or rcu_sched
states and instead will be operating on rcu_bpf per-cpu states.
In theory that should have worked flawlessly and it sort-of did.
But multiple hackbench runs still managed to crash it.
So far I think the temp workaround is to stick with array maps
for probing such low level things like trace_preempt.
Note that pre-allocation of all elements in hash map also won't
help, since the problem here is some collision of call_rcu and
rcu_process_callbacks. I'm pretty sure that kfree_rcu with
rcu_is_watching patch is ready for this type of abuse.
The rcu_process_callbacks() path - no yet. I'm still analyzing it.

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