Bug 10013

One way to implement this could be to add a word-sized counter to the
.probes section that describes the probe point.  Translator-generated
code could attempt to atomically increment/decrement that when the
corresponding uprobe is placed/removed.  The ENABLED() macro would read that
value.

One way to make this somewhat less flaky/racy is to have all the counters
be set/cleared on a object-file-wide basis, when task-finder tells us it has
been loaded -- so before the user-space code starts to run.

Maybe we can teach uprobes to replace with a specific move/assignment
instruction instead of a trap instruction. Then you could define ENABLE_PROBE()
with something like:

#define ENABLED_PROBE() ({char e = 0; asm ("mov $0x0, %0" :"=r"(e):"0"(e)); e;})

(of course with the magic macro label .probe section stuff to know the actual
address of the mov instruction)

Then instead of with a normal STAP_PROBE replacing a "nop" with "int3", you
would replace the "mov $0x0, %somereg" with "mov $0x1, %somereg".

> Maybe we can teach uprobes to replace with a specific move/assignment
> instruction instead of a trap instruction.

This optimization is the same as in the "move immediate" kernel markers.
However, it is not a *substitute* for a trap.  It is only a substitute
for a condition evaluation.

> Then instead of with a normal STAP_PROBE replacing a "nop" with "int3", you
> would replace the "mov $0x0, %somereg" with "mov $0x1, %somereg".

You still need some way to enter the kernel.

(In reply to comment #3)
> This optimization is the same as in the "move immediate" kernel markers.
> However, it is not a *substitute* for a trap.  It is only a substitute
> for a condition evaluation.
> [...]
> You still need some way to enter the kernel.

It isn't meant as substitute of the trap. That will still be the STAP_PROBE itself.

So the idea is that when enabling a probe, you first enable the PROBE itself,
replacing the nop with a trap, then, if there is also an associated ENABLED
conditional, you replace the move to enable the code path leading to the probe.
So user code would be written as:

if (STAP_PROBE_ENABLED(myapp, mymark))
 {
   do_expensive_argument_setup();
   STAP_PROBE(myapp, mymark, args);
 }

commit: 63b4fd14
Adds process("process").library("library").mark("mark")
And uses it, via the task manager, to implement enabled.