On Tue, 2 Feb 2021 11:45:41 +0100
Peter Zijlstra [off-list ref] wrote:

quoted

The stack tracer checks the size of the stack, compares it to the
largest recorded size, and if it's bigger, it will save the stack. But
if this happens on two CPUs at the same time, only one can do the
recording at the same time. To synchronize this, a spin lock must be
taken. Similar to spin locks in an NMI.  

That sounds like something cmpxchg() should be able to do.

Have a per-cpu stack trace buffer and a global max one, when cpu local
exceeds previous max, cmpxchg the buffer.

quoted

But the problem here is, the callbacks can also be done from an NMI
context, so if we are in NMI, we don't want to take any locks, and
simply don't record the stack traces from NMIs.  

Which is obviously shit :-) The NMI might have interesting stack usage.

Actually, it only checks task stacks. It doesn't check IRQ stacks if
they are different than the task stack, because to do it properly, it
must know the size of the stack. The tracer currently masks the stack
pointer with THREAD_SIZE to find the top of the stack.

As other stacks may not be THREAD_SIZE, that won't work. It has been on
my TODO list (for a long time), to add an arch specific way to quickly find
the top of the stack.

quoted

The more I think about it, the more I hate the idea that ftrace
callbacks and kprobes are considered NMIs. Simply because they are not!  

Yet they happen when IRQs are off, so they are ;-)

But from a handler, you could do:

	if (in_nmi())
		return;
	local_irq_save(flags);
	/* Now you are safe from being re-entrant. */

Where as there's no equivalent in a NMI handler. That's what makes
kprobe/ftrace handlers different than NMI handlers.

Also, given how everything can nest, it had better all be lockless
anyway. You can get your regular function trace interrupted, which can
hit a #DB, which can function trace, which can #BP which can function
trace again which can get #NMI etc.. Many wonderfun nestings possible.

I would call #DB an #BP handlers very special.

Question: Do #DB and #BP set "in_interrupt()"? Because the function tracer
has infrastructure to prevent recursion in the same context. That is, a
ftrace handler calls something that gets traced, the recursion protection
will detect that and prevent the handler from being called again. But the
recursion protection is interrupt context aware and lets the handler get
called again if the recursion happens from a different context:

func:
   call ftrace_caller
      ftrace_caller:
          call ftrace_handler
              ftrace_handler() {

                   if (recursion_test()) <- false
                       return;

                   some_traced_func() {
                        call ftrace_caller
                               call ftrace_handler
                                    ftrace_handler() {
                                        if (recursion_test()) <- true
                                               return
                                    }
                    <interrupt>
                         func
                            call ftrace_caller
                                 call ftrace_handler
                                       ftrace_handler() {
                                            if (recursion_test()) <- false
                                                 return;
                                       /* continue */


If #DB and #BP do not change the in_interrupt() context, then the above
still will protect the ftrace handlers from recursion due to them.

And god knows what these handlers end up calling.

The only sane approach is treating it all as NMI and having it all
lockless.

That would require refactoring all the code that's been around since 2008.

Worse yet. lockless is much more complex to get right. So this refactoring
will likely cause more bugs than it solves.

-- Steve