On Wed, 12 Aug 2020 22:13:19 -0700 (PDT)
Palmer Dabbelt <pal...@dabbelt.com> wrote:
> Sorry, I'm not really sure what's going on here. I'm not really seeing code
> that matches this in our port right now, so maybe this is aginst some other
> tree? If it's the RISC-V kprobes patch set then I was hoping to take a look
> at
> that tomorrow (or I guess a bit earlier this week, but I had some surprise
> work
> stuff to do). IIRC there were a handful of races in the last patch set I saw,
> but it's been a while so I don't remember for sure.
>
> That said, I certainly wouldn't be surprised if there's a locking bug in our
> ftrace stuff. It'd be way easier for me to figure out what's going on if you
> have a concrete suggestion as to how to fix the issues -- even if it's just a
> workaround.
The issue is actually quite basic.
ftrace_init_nop() is called quite early in boot up and never called
again. It's called before SMP is set up, so it's on a single CPU, and
no worries about synchronization with other CPUs is needed.
On x86, it is called before text_poke() is initialized (which is used
to synchronize code updates across CPUs), and thus can't be called.
There's a "text_poke_early()" that is used instead, which is basically
just a memcpy().
Now, if ftrace_init_nop() is not defined by the architecture, it is a
simple call to ftrace_make_nop(), which is also used to disable ftrace
callbacks.
The issue is that we have the following path on riscv:
ftrace_init_nop()
ftrace_make_nop()
__ftrace_modify_call()
patch_text_nosync()
patch_insn_write()
lockdep_assert_held(&text_mutex);
Boom! text_mutex is not held, and lockdep complains.
The difference between ftrace_make_nop() being called by
ftrace_init_nop() and being called later to disable function tracing is
that the latter will have:
ftrace_arch_code_modify_prepare();
[..]
ftrace_make_nop();
[..]
ftrace_arch_code_modify_post_process();
and the former will not have those called.
On x86, we handle the two different cases with:
static int ftrace_poke_late = 0;
int ftrace_arch_code_modify_prepare(void)
{
mutex_lock(&text_mutex);
ftrace_poke_late = 1;
return 0;
}
int ftrace_arch_code_modify_post_process(void)
{
text_poke_finish();
ftrace_poke_late = 0;
mutex_unlock(&text_mutex);
}
Although, the post_process() probably doesn't even need to set
ftrace_poke_late back to zero.
Then in ftrace_make_nop(), we have:
ftrace_make_nop()
ftrace_modify_code_direct()
if (ftrace_poke_late)
text_poke_queue(...); // this checks if text_mutex is held
else
text_poke_early(...); // is basically just memcpy, no test on
text_mutex.
The two solutions for riscv, is either to implement the same thing as
above, or you can create your own ftrace_init_nop() to take the
text_mutex before calling ftrace_make_nop(), and that should work too.
-- Steve
