Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

[Andy Lutomirski]

On Thu, Jan 22, 2015 at 1:52 PM, Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> On 11/21/2014 04:26 PM, Andy Lutomirski wrote:
>> We currently pretend that IST context is like standard exception
>> context, but this is incorrect.  IST entries from userspace are like
>> standard exceptions except that they use per-cpu stacks, so they are
>> atomic.  IST entries from kernel space are like NMIs from RCU's
>> perspective -- they are not quiescent states even if they
>> interrupted the kernel during a quiescent state.
>>
>> Add and use ist_enter and ist_exit to track IST context.  Even
>> though x86_32 has no IST stacks, we track these interrupts the same
>> way.
>>
>> This fixes two issues:
>>
>>  - Scheduling from an IST interrupt handler will now warn.  It would
>>    previously appear to work as long as we got lucky and nothing
>>    overwrote the stack frame.  (I don't know of any bugs in this
>>    that would trigger the warning, but it's good to be on the safe
>>    side.)
>>
>>  - RCU handling in IST context was dangerous.  As far as I know,
>>    only machine checks were likely to trigger this, but it's good to
>>    be on the safe side.
>>
>> Note that the machine check handlers appears to have been missing
>> any context tracking at all before this patch.
>
> Hi Andy, Paul,
>
> I *suspect* that the following is a result of this commit:
>
> [  543.999079] ===============================
> [  543.999079] [ INFO: suspicious RCU usage. ]
> [  543.999079] 3.19.0-rc5-next-20150121-sasha-00064-g3c37e35-dirty #1809 Not tainted
> [  543.999079] -------------------------------
> [  543.999079] include/linux/rcupdate.h:892 rcu_read_lock() used illegally while idle!
> [  543.999079]
> [  543.999079] other info that might help us debug this:
> [  543.999079]
> [  543.999079]
> [  543.999079] RCU used illegally from idle CPU!
> [  543.999079] rcu_scheduler_active = 1, debug_locks = 1
> [  543.999079] RCU used illegally from extended quiescent state!
> [  543.999079] 1 lock held by trinity-main/15058:
> [  543.999079] #0: (rcu_read_lock){......}, at: atomic_notifier_call_chain (kernel/notifier.c:192)
> [  543.999079]
> [  543.999079] stack backtrace:
> [  543.999079] CPU: 16 PID: 15058 Comm: trinity-main Not tainted 3.19.0-rc5-next-20150121-sasha-00064-g3c37e35-dirty #1809
> [  543.999079]  0000000000000000 0000000000000000 0000000000000001 ffff8801af907d88
> [  543.999079]  ffffffff92e9e917 0000000000000011 ffff8801afcf8000 ffff8801af907db8
> [  543.999079]  ffffffff815f5613 ffffffff9654d4a0 0000000000000003 ffff8801af907e28
> [  543.999079] Call Trace:
> [  543.999079] dump_stack (lib/dump_stack.c:52)
> [  543.999079] lockdep_rcu_suspicious (kernel/locking/lockdep.c:4259)
> [  543.999079] atomic_notifier_call_chain (include/linux/rcupdate.h:892 kernel/notifier.c:182 kernel/notifier.c:193)
> [  543.999079] ? atomic_notifier_call_chain (kernel/notifier.c:192)
> [  543.999079] notify_die (kernel/notifier.c:538)
> [  543.999079] ? atomic_notifier_call_chain (kernel/notifier.c:538)
> [  543.999079] ? debug_smp_processor_id (lib/smp_processor_id.c:57)
> [  543.999079] do_debug (arch/x86/kernel/traps.c:652)
> [  543.999079] ? trace_hardirqs_on (kernel/locking/lockdep.c:2609)
> [  543.999079] ? do_int3 (arch/x86/kernel/traps.c:610)
> [  543.999079] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2554 kernel/locking/lockdep.c:2601)
> [  543.999079] debug (arch/x86/kernel/entry_64.S:1310)

I don't know how to read this stack trace.  Are we in do_int3,
do_debug, or both?  I didn't change do_debug at all.

I think that nesting exception_enter inside rcu_nmi_enter should be
okay (and it had better be, even in old kernels, because I think perf
does that).

So you have any idea what you (or trinity) did to trigger this?

--Andy
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