[PATCH] powernv: Avoid calling trace tlbie in kexec path.

[Michael Ellerman]

Balbir Singh <bsingharora at gmail.com> writes:

> On Thu, Nov 23, 2017 at 4:32 AM, Mahesh J Salgaonkar
> <mahesh at linux.vnet.ibm.com> wrote:
>> From: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
>>
>> Rebooting into a new kernel with kexec fails in trace_tlbie() which is
>> called from native_hpte_clear(). This happens if the running kernel has
>> CONFIG_LOCKDEP enabled. With lockdep enabled, the tracepoints always
>> execute few RCU checks regardless of whether tracing is on or off.
>> We are already in the last phase of kexec sequence in real mode with
>> HILE_BE set. At this point the RCU check ends up in RCU_LOCKDEP_WARN and
>> causes kexec to fail.
>>
>
> Effectively we can't enter the trace point code after we've set
> HILE_BE.  Do we need
> a fixes tag? Or is this a side-effect of a new generic change?

Yes I added:

Fixes: 0428491cba92 ("powerpc/mm: Trace tlbie(l) instructions")
Cc: stable at vger.kernel.org # v4.13+

> I think the right thing in the longer run might be to do a TRACE_EVENT_CONDITION
> and have the condition do the right thing, but what you have for now is good.

No I think the right thing is to not call trace points from kexec code,
it's too fragile. TRACE_EVENT_CONDITION wouldn't have saved us from this
RCU breakage.

cheers