[PATCH] ftrace: Fixup lockdep assert held of text_mutex
palmer at dabbelt.com
Mon Aug 24 20:29:32 EDT 2020
On Thu, 13 Aug 2020 08:37:43 PDT (-0700), rostedt at goodmis.org wrote:
> On Wed, 12 Aug 2020 22:13:19 -0700 (PDT)
> Palmer Dabbelt <palmer at 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
> 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
> The issue is that we have the following path on riscv:
> 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:
> 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)
> ftrace_poke_late = 1;
> return 0;
> int ftrace_arch_code_modify_post_process(void)
> ftrace_poke_late = 0;
> Although, the post_process() probably doesn't even need to set
> ftrace_poke_late back to zero.
> Then in ftrace_make_nop(), we have:
> if (ftrace_poke_late)
> text_poke_queue(...); // this checks if text_mutex is held
> 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.
Ya, thanks, that's a pretty straight-forward issue. I've To'd you on a patch,
but it's essentially just exactly what you suggested so I doubt it's that
I pointed out in the patch notes that it seems reasonable to have the generic
code handle this case, would you be opposed to doing it that way?
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