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

[Mahesh Jagannath Salgaonkar <mahesh@linux.vnet.ibm.com>]

On 11/23/2017 12:37 AM, Naveen N. Rao wrote:
> Mahesh J Salgaonkar wrote:
>> From: Mahesh Salgaonkar <mahesh@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.
>>
>> Fix this by not calling trace_tlbie() from native_hpte_clear().
>>
>> Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
>> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>  arch/powerpc/mm/hash_native_64.c |   15 ++++++++++++---
>>  1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/hash_native_64.c
>> b/arch/powerpc/mm/hash_native_64.c
>> index 3848af1..640cf56 100644
>> --- a/arch/powerpc/mm/hash_native_64.c
>> +++ b/arch/powerpc/mm/hash_native_64.c
>> @@ -47,7 +47,8 @@
>>
>>  DEFINE_RAW_SPINLOCK(native_tlbie_lock);
>>
>> -static inline void __tlbie(unsigned long vpn, int psize, int apsize,
>> int ssize)
>> +static inline unsigned long  ___tlbie(unsigned long vpn, int psize,
>> +                        int apsize, int ssize)
>>  {
>>      unsigned long va;
>>      unsigned int penc;
>> @@ -100,7 +101,15 @@ static inline void __tlbie(unsigned long vpn, int
>> psize, int apsize, int ssize)
>>                   : "memory");
>>          break;
>>      }
>> -    trace_tlbie(0, 0, va, 0, 0, 0, 0);
> 
> Does it help if you use the _rcuidle variant instead, to turn off all
> rcu checks for tracing __tlbie()?
>     trace_tlbie_rcuidle(0, 0, va, 0, 0, 0, 0);

It helps if tracepoint is not enabled. But with tracepoint enabled kexec
still fails. I think we should not have tracepoint in kexec path at all.
If someone enables it, kexec will definitely fail regardless of
CONFIG_LOCKDEP.

Thanks,
-Mahesh.