Re: [PATCH 4/7] powerpc/ftrace: Additionally nop out the preceding mflr with -mprofile-kernel

[Nicholas Piggin <npiggin@gmail.com>]

Michael Ellerman's on June 19, 2019 3:14 pm:
> Hi Naveen,
> 
> Sorry I meant to reply to this earlier .. :/
> 
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> writes:
>> With -mprofile-kernel, gcc emits 'mflr r0', followed by 'bl _mcount' to
>> enable function tracing and profiling. So far, with dynamic ftrace, we
>> used to only patch out the branch to _mcount(). However, mflr is
>> executed by the branch unit that can only execute one per cycle on
>> POWER9 and shared with branches, so it would be nice to avoid it where
>> possible.
>>
>> We cannot simply nop out the mflr either. When enabling function
>> tracing, there can be a race if tracing is enabled when some thread was
>> interrupted after executing a nop'ed out mflr. In this case, the thread
>> would execute the now-patched-in branch to _mcount() without having
>> executed the preceding mflr.
>>
>> To solve this, we now enable function tracing in 2 steps: patch in the
>> mflr instruction, use synchronize_rcu_tasks() to ensure all existing
>> threads make progress, and then patch in the branch to _mcount(). We
>> override ftrace_replace_code() with a powerpc64 variant for this
>> purpose.
> 
> According to the ISA we're not allowed to patch mflr at runtime. See the
> section on "CMODX".

According to "quasi patch class" engineering note, we can patch
anything with a preferred nop. But that's written as an optional
facility, which we don't have a feature to test for.

> 
> I'm also not convinced the ordering between the two patches is
> guaranteed by the ISA, given that there's possibly no isync on the other
> CPU.

Will they go through a context synchronizing event?

synchronize_rcu_tasks() should ensure a thread is scheduled away, but
I'm not actually sure it guarantees CSI if it's kernel->kernel. Could
do a smp_call_function to do the isync on each CPU to be sure.

Thanks,
Nick